Table Of ContentNOTES
ON .
The Autotype
Carbon Process
BY
A. C. BRAHAM, . F.R.P.S.
Chief of the Technical Staff
at
The Au.totype Works.
W'est Ealing.
~
~
PUBLISHED BY
THE AUTOTYPE COMPANY,
74 New Oxford St., London·,
(and West Ealing) .
•
PRICE SIXPENCE.
FOREWORDS.·
THE following chapters were originally published in the
Australian Photo Review at Melbourne, and in Snap Shots
at New York, in the hope of increasing the use of the
Carbon process abroad.
They are now issued m the home of carbon printing,
with the wish to convince beginners that the process is as
easy as it is desirable, and in the hope that they may be
of assistance both to the beginner and the regular user
in indicating the steps which most surely lead to success
in working.
A. C. BRAHAM.
WEST EALING,
Marek, 1908.
,'.. ;
THE CARBON PROCESS.
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CHAPTER !.-INTRODUCTION.
IT is strange that carbon printing should be neglected by
many amateurs, who while willing to take great pains in the
selection of subjects for camera work, and produce excellent
negatives are then content to employ printing processes which
are only a second best ; neglecting a process which will '' give
a greater variety of effect than any other process or indeed than
all the other printing processess combined.''
This comment, extracted from an editorial in the British
Journal of Photography, certainly shows a high appreciation
of the merits and advantages of carbon printing ; an appre
ciation which may however readily be substantiated.
Should the subject represented be a Cathedral interior,
various shades of black may be employed, Brown Black or
Warm Black developed on slightly toned transfer paper
giving a charming rendering of stonework; should the choir,
or det~ils of stalls or other woodwork be represented, various
browns, such as Cool Brown . or Sepia are more usually
employed.
Landscapes may be printed in blacks,·b rowns, and greens,.
of which colour several different tones are manufactured; in
fact, a colour chart issued by the Autotype Company contains
examples of thirty distinct standard colours, and even others.
are made to meet special requirements.
2 3
For those fortunateiy enough situated to be able to take have a reasonable certainty that the result shall be per
snow scenes, Engraving Black (printed lightly) offers a means manent.
This may be secured by employing carbon prints; the
of rendering the range of gradation in snow, between a sunlit
best makers of tissue being scrupulously careful to use only
patch and the shadows in a drift not otherwise to be obtained.
For sea-scapes, snap shots of yachts, studi~s of breakinO' permanent pigments in the manufacture of their products,
waves, the. various greens and blues ranging~ from Italia~ and to use papers that are least liable to change tone with
age, for all papers, even the best, are subject to a slight but
Green to Turquoise Blue will give a choice of· colours wide
enough to satisfy the most exacting taste. -' negligable change of tone. .
The choice of a process is often decided by the question
In Portraiture, so called ".photographic tones " are readily
of cost, although economy should be rather a wise expenditJ.Ire
obtained by using-Portrait Brown, Ruby Brown or Warm
of money. than merely a question of cheapness. In this
Sepia ; . many profes~ional photographers . not bound by
respect, carbon is about on a par with silver printing because
convent10n, employ with great success, especially for portraits
although the cost of tissue and transfer may be slightly above
of ladies and children, Red Chalk and Terra Cotta, and
that of silver paper, there is no further incidental expense for
occasionally even Bright Red or Carmine. . .
Reproductions of paintings, drawings, etc., may be made toning solutions to be considered.
Compared with the only other permanent process, platinum,
in the pigments of the Artist's palette-hence the name
carbon including tissue and transfer paper, will be found to
'' AuT~TYPE,". which was first used by the. late Tom Taylor
be only half the price of platinum paper, taking the price
(som~time Ed1t?r of Punch, and art critic) to define repro
from the current list of the leading firm making platinum
d~ctions of .artist 1s work, made photographically, that is,
paper. This extreme difference of price is partly accounted
w1th~ut the mtervention of another's eye or hand; the term
for by the. present high price of the metal platinum, but at
~as smce been ~xtended to includ~ any photograph produced
m permanent pigments. · · all times carbon is far less costly. ·
Having dealt with the beauty and variety of the process,
~y the choice of suitable transfer papers endless modifi
with its important advantage of permanency and its cost,
cations of tone and texture may be obtained,' ranging from
I will proceed to. the practical details of manipulation, bearing
the extreme gloss of P. 0. P. (should such be desired) to the
in mind Dr. Jo hnson's phrase from the Introduction to his
deep dead matt of a fine mezzotint engraving, and with an
Dictionary, "Words are the daughters of earth, but things
imperceptible grain or the roughest texture. ·
are the sons of heaven."
Who that ?W~~ photographs of t?ose "Loved long since
and lost awhile, has .not had occas10n to lament the fading
of so.me valued portr.a1t. ? and from a merely utilitarian point
of v1~w what. folly_ 1t 1s to spend either time or money in
workmg up, either m colour or monochrome on a base which CHAPTER II.-GENERAL PRINCIPLES.
will. probably become ~ellow and fade, dest;oying the values
THE practice of. carbon printing depends on the property of
of bght and shade obtamed, and making what was a pleasing
portrait only grotesque. ~elatine, combined with bichromate of Potassium, becoming
!~soluble when exposed to light. If carbon or any suitable
For t~e credit of photography it is to be regretted that p1gm~nt b~ incorporated with the gelatine, the insoluble
the pubbc do not always realise the risky nature of a·worked
gelatme will hold and imprison the colouring matter. If
up silv~r print, and insist that when they pay a certain or a sheet of paper which has been coated with gelatine mixed
1.mcertam number of guineas for artistic finishing, they shall
4 5
with pigment and bichromate and allowed to dry, be exposed MATERIALS AND APPARATUS NECESSARY.
to light under a negative, varying proportions of the gelatine For those working on an extensive scale or makin~ large
will be rendered insoluble. Through the shadows of the prints developing tanks may be needed and a special ~ot
negative where much light passes, much gelatine will conse water, supply ; but these cannot ~e regarded as a necessity
quently be .rendered insoluble-through the h~lf tones less for the smaller worker. Any ordmary tray or vessel, as the
light passes-less gelatine therefore becomes insoluble, through .domestic bath or basin and kettle will serve the P1:rpo~
the highest lights of the negative practically no light passes, the squeegee and actinometer being the o~ly. pieces of
consequently the whole of the gelatine remains soluble. The apparatus ~bsolutely ess~ntial beyond the pnntmg frames
image thus formed"in the tissue (as the coated paper is called) and dishes m the possession of any photographer. '. .
can now be revealed by dissolving and was:µ.ing away the The insensitive tissue cut to pieces of the reqmred size,
remaining soluble gelatine and colouring matter. The for must be sensitized by immersion in a solution of bichron;ate
mation of a carbon print is therefore analogous,, although -0f potash-allowed to dry in the .dark an~ ~xpos~d· tq light
exactly the reverse process, to making a wash drawing. In under a negative. As there is no vlSlble _imag~, an
the case of a wash drawing we start with clean paper and by actinometer is used to gauge the exposure which will_ be
adding successive washes of colour build up the image-in approximately ! t~ ! of !hat need~d for a P. 0: P. prmt.
the case of a carbon print we start with paper heavily coated The exposed tissue is then immersed m cold water (m ~corner
with pigment, form an image by the action of light' and reveal ·of a room removed from bright light) together with the
it by washing away the superfluous colour that' hides it. transfer paper, opal or other base it is intende~ to be devefoped
Pioneers in carbon printing endeavoured to develop the upon. In the cold water the tissue ~ill cu~l 1tself up more. or
image by immersion in hot water-but success was only less according to the amount of moisture it already contams
obtained when it was recognised that the insoluble image ·was -just before it becomes flat or flaccid it is remo"."'e? from the
upon or on the upper portions of the pigment coating, the water together with its support, placed on a ngid surface,
soluble portions being the lower parts or those in contact with and the squeegee passed lightly and evenly over the back of
the paper in which the coating was spread. The dissolving the tissue to expel superfluous water and secure contact
therefore must take place from the back, and, means were between the two surfaces. The adherjng tissue and transfer
then sought to attach the printed tissue to a support. Various .are next placed between blotting boards and allo~ed to
cements, among them India rubber solution were used; both remain about ten minutes, after which the operation of
tissue and support being coated with-the solution, allowed to development may take place in full daylight; which is an
partially dry and then passed in contact through the rolls of .advantage in that it allows the picture t<? be ~ev:elop~d and
a copper-plate press. Under these conditions it is not sur the correct depth and effect to be judged m a similar light to
prising that carbon printing was regarded as for the few. that in which it will afterwards usually be seen.
Soon, however, the cementing of the tissue was abolished, Development is commenced by taking the exposed tissue
transfer paper sold ready coated with insoluble gelatine, and adhering to its support and placing in w:arm water at a.tempera
the costly and cumbrous copper-plate press replaced by ture just ~omfortable to the hand, m about a mmute. the
a handy squeegee costing only a few pence. -soluble gelatine and pigment will become softened and
I have allowed myself this historical digression believing dissolve and will be noticed oozing out at the edges. The
not only that it is of interest, but being convinced that an paper on which the tissue was made is then lifted by the
understanding of past difficulties and errors is a great help finger nail from one corner under water and pulled away from
to present success and ease of work. the support-the insoluble image hidden with some soluble
6 7
tissue remaining on the support for development. Water is . CHAPTER IIl.-SENSITIZING THE TISSUE.
now splashed on this, using the right hand as a sort of cup~
It may. be found a convenience to lay the embryo print face CARBON tissue in its insensitive condition will keep for pro
upwards on a floating wooden bo;u:d to keep it rigid. The longed periods, consideration being given to the fact that it
development of carbon prints is considerably urnier control- is paper coated with a gelatinous compound and consequently
hotter water: increasing the conti:ast-the hot ~-water being liable under conditions of extreme humidity to the growth
either splashed with the hand gen_erally over ~he print · or of mould-it should, therefore, be stored in a dry place.
poured on any part of the print it is desired to lighten locally. Tissue may be bought in fiat packets of cut pieces in
When development is judged complete, the picture is rinsed standard sizes, the user of large quantities or irregular sizes
in cold water to stop further action, and then put into a 5 per will prefer, however, to buy it in rolls or, as they are technically
cent. solution of alum to -harden the gelatine and to remove· called, bands; a term indicating the form in which the tissue
any traces of biChromate remaining in the paper or image. is made and exists previous to rolling.
When the light yellow bichromate stain has disappeared the If the atmosphere is in an extremely dry state the roll of
picture is rinsed in cold water to remove any scum--from the tissue may be slightly brittle and inclined to crack when
alum solution, hung up and allowed to dry spontaneously. unrolled-it should, in that case, be put in a damp place for
The pic'b'ure then, presuming it has been developed in single· a short time previous to cutting it up.
transfer, is finished. If the hands are disposed to perspire freely it is advisable
Single transfer is available (from negatives taken on glass to wear cotton gloves while cutting up the tissue as finger
direct in the camera) tor such subjects as are not, affected by marks would show their impression in the resulting prints,
inversion as to right and left handedness such as wave studies, and however interesting, curious, and valuable the Bertillon
and for landscapes and figures that are intended primarily~as. system may be in criminal records it is unwelcome and
pictures; where they are intended as topographical' records wasteful in other photographs. The wearing of gloves also
either of scenery or buildings, double transfer must be prevents accidental scratching of the tissue surface by the
employed. If, however, film negatives made on' thin celluloid finger nails. 1
are employed, they may be printed from either side, 'no loss. SENSITIZING THE TISSUE:-
of sharpness being visible to the most critical eye~ · It is essential to use pure bichromate of potassium, that
Where double transfer is essential the print is developed used for batteries and other industrial purposes and
on a temporary sl!pport.\fhich may be either rigid as a piece purchasable at the oil shop is to be avoided, bought from
of waxed matt opal, giving an egg-shell surface to the finished a photographic dealer or chemist, it will be satisfactorily
print or on Sawyer's flexible support; a paper support which pure; if any doubt be felt on the question, add sufficient
has been coated with _an aqueous solution of lac and requires,. liquid ammonia, after dissolving the bichromate, to change
before use, to be rubbed over with a solution of resin and ·bees the solution from a reddish to a decided yellow colour; the
wax in turpentine, known as waxing solution. 'fhis gives. granulated bichromate prepared by the Autotype Co. will be
a· surface which holds the picture during development, but found constant and reliable. ·
allows it readily to leave its temporary support when a sheet The bichromate may be dissolved by shaking it up)n cold
of final support has been squeegeed upon it and allowed to dry"'. water, but a more generally convenient method is to tie the
This flexible support gives to· the print a smooth slightly desired weight of bichromate loosely in a piece of muslin to
glazed surface very like that of an albumenized print. form a nag and suspend it in a jug of hot water containing
one pint of water for each ounce of bichromate employed,
8 9
when cool the solution, may be bottled and kept for use, rnismg any trouble with air bells and in permitting the tissue
being practically a five per cent. solution. to curl without its edges coming above the surface of the
The strength of . the sensitizing solution desirable for solution. The temperature of the sensitizing solution should
normally dense negatives may -~be taken as four per cent., not exceed 70° F. Ice being used if required, as at higher
considerable.control is practicable in development in modifying temperature the coating is liable to melt.
or exaggerating ·contrast-but for very thin , negatives a One minute's immersion in the bath will be found long
weaker sensitizing solutfon is advantageous. A weaker bath enough to allow the tissue to absorb sufficient of the sensitizing
giving a lower sensitiveness to the tissue-tissue sensitized in solution and to ensure its equal diffusion through the body
a one per, cent. solution requiring three to four times .the of the tissue.
exposure needed by that sensitized in a five per cent. bath. Longer immersion is undesirable as continued soaking,
The five per cent. solution is a convenient standard strength beside making the tissue less easy to dry, dissolves some of
to dilute as required. the sugar introduced in the manufacture for the double
20 ozs. Standard solution+ 5 ozs. water giving 4 % solution. purpose of preventing liability to brittleness in dry weather,
20 " + 13 " 3 % and to render the tissue more porous during development.
+ % Remove the tissue from the solution, drawing the back over
20 " 20 " 2!
+ % " the edge of the dish to get rid of superfluous moisture, and
20 " " " 30 " " " 2
The sensitizing solution may be used several times, being hang it up to dry, by one corner if small, by two if of large
kept when not in use in bottles stored in the dark, as the size.
decomposition of the solution is then less rapid. It is not It may eitherJbe pinned to a lath (the glass headed pins
possible to fix the exact number of times the solution may be used for bromides being handy) or suspended by wooden
used, as more depends on the periods elapsing between use clips hanging from a string, but a more convenient method
than on the amount of tissue passed through it. A -too old is to prepare some laths by driving brass brads through the
bath will give impaired solubility. · edge of them and hanging· the tissue on to the projecting
An earthenware developing dish or tin tray having a smooth points of the brads.
beaded edge will be convenient to contain the sensitizing The sensitizing may be done in any artificial light, but the
solution of which there should be at least one inch in depth, drying must be in the dark, or a yellow light, and should be
and preferably more. · -~ complete in at most six hours-long delayed drying being
The cut pieces of tissue (their being irt a flat condition from likely to cause insolubility of the tissue.
having been kept under slight pressure will make them more High temperature, that is, over 80°, is not desirable (at
convenient to handle) should be lifted by the two corners least in the early stages; although a higher temperature is
further from oneself and drawn face upwards under the permissible when the tissue is nearly dry), but rather dry and
solution from the near edge of the dish towards the further moving air; it is preferable to dry in a room with ample air
side, over the edge of which it is advisable to draw or lightly space; but if a drying cupboard is used care must be taken
drag the back of the tissue to remove any air bells formed that the circulating air is pure. When electricity is available,
thereon-the tissue should be returned to the solution still one or two small electric fans-such as are used on the table
face upwards, the dish being gently rocked to remove any air in the summer months-prove an excellent means of keeping
bells formed on the face, as if air bells be allowed to remain the air in motion.
they will cause insensitive spots in the tissue ; the advantage The tissue should be dried thoroughly, a condition indicated
.of having sufficient depth of solution will be found in mini- by its curling up and feeling quite crisp, and should then be
10
II
stored in a tin tube with a well-fitting lid. It will, thus will be somewhat less than when an aqueous solution of tb~
stored, keep in a good soluble condi~ion for from one to three same strength is used.
weeks-for tissue once sensitized is. i'n an unstable condition,
starting with normal sensitiveness' and complete -solubility it .. FoR UsE.
passes gradually (in a period varying with the temperature Pour sufficient of the sensitizer into a small saucer or other
and degree of moisture present in the atmosphere) through shallow vessel to the depth of about an eighth of an inch.
lessening solubility to ultimately complete insolubility. This ~'A Blanchard Brush" (consisting of a piece of flannelette
change is due entirely to the presence of mqisture, with fixed by an elastic band in a double fold over the end of
complete dessication it is a reasonable assumption that a glass strip) is then dipped in the solution and passed lightly,
solubility would be indefinitely pre~erved. . with a movement between that of a brush and a squeegee,
Complete dessication is, however, a state in which it would over the face of the tissue (laid on a clean piece of paper) to
not be practicable to use the tissue; a degree of' dryness evenly distribute the solution.
sufficient to preserve the tissue certainly three mont~s either If the "brush" be passed lengthwise and then crosswise
before printing or between printing and development may be over the tissue even sensitizing will' be secured, the brush
secured by using a special storage box consisting of a japanned having been redipped in the sensitizer to keep it tnode~ately
tin box, so made as to keep the tissue flat under slight pressure, wet. Too dry a brush might mark the surface of the tissue,
provided with a well-fitting lid and a draw contai~ing pellets too wet cause uneven absorption. .
of asbestos fibre and calcium chloride over which any air After three or four pieces have been thus treated, the fir~t
entering the box must pass before it reaches the tissue, the piece brushed will be surface dry and should t~en b~ agam
calcium chloride absorbing any moisture present in the air, treated in the same manner, this second coatmg with the
and preventing its access to the tissue, which may thus be sensitizer will give increased sensitiveness. . .
kept in good condition for several months, this box is a con The tissue suspended in a warm room will dry m about
venience in all cases and particularly to those whose use of ten minutes, it may even without damage be held close enm~gh
tissue is only occasional. to a fire to dry in two or three minutes, but the slower drymg
is the safer procedure.
AUTOTYPE. SPIRIT SENSITIZER. The tissue may be used as soon as it is sufficien~ly dry t?
... be safe to put upon an unvarnished gelatine negative,. but ~f
The Autotype Co., realising the unfavourable conditions
it is intended to keep the tissue it is necessary to dry it until
for drying tissue under which workers labour, and the con
it curls and feels crisp. It can then be kept for months
venience of being able to sensitize and dry tissue within a few
equally well with tissue sensitized in the ordinary aqueous
minutes, have introduced their Spirit Sensitizer.
sensitizing solution by the use of a storage box. . ..
All tissue to ensure there being sufficient tissue compound
The Spirit Sensitizer can, if desired, be used for se~sitizmg
or pigmented gelatine in the thinnest part of the tissue, has
by immersion, but the time of drying will then be conside:ably
to receive a thicker coating than is utilised in making a print.
longer and the sensitiveness greater than with t~e brushed t~s~ue.
The Spirit Sensitizer being applied to the front of the tissue,
Immersed tissue (due to the larger quantity of sensit~zer
although it will not penetrate right through will soak deeply
absorbed) will be found to increase in sensitiveness by ke~pmg.
enough into ·the body of the tissue to sensitize a sufficient
It is necessary that the sensitizing bath when not m use
proportion of the pigmented gelatine for printing from. the should be kept in the dark, as it is liable to change by the
large majority of negatives. The sensitiveness of the tissue action of light.
13
12
I am only able to speak personally of the practice of carbon "It consists of a well made wooden b'ox o; convenient
printing under the conditions of the English climate, which dimensions, according to the size of tissue and with a closely
by general admission are sufficiently cvaried. fitting lid. The chloride of calcium is contained in an iron
C~ncerning the pr~ctice of carbop ·printing inr Jhe tropics, <lish at the bottom of the box, and should·b e taken out and
I will quote the high authority of General \Vaterhouse dried over a fire once or twice a week or as necessary to keep
late Assistant Surveyor General in charge of the Photographi~ up the drying power of the chloride."
Department, Calcutta, who writes ih a pamphl~t on the l have, in describing the methods of working, pointed out
proc~ss. " By t~king the necessary precautions, it is quite possible causes of failure or difficulty, not thinking them all
possible to work it all -the year round under ordinary tropical likely to arise, but as we all, in the absence of experience>
conditions." have a fatal facility for finding wrong methods of proceeding,
T~e difficulties, caused by extrein:es of temperature, are I have e~dea voured to warn against them beforehand.
not hkely to ~e ~xperienced principally in printing or develop
ment, that is, m the use of the tissue, but rather in hts
sensitizin~ and d~ying, because when the temperature is high
and the ?-Ir .lleav1ly charged with moisture there is a danger CHAPTER IV.-PRINTING.
of the tissue softening in the warm sensitizing bath and
partly me~ting and running during drying. · THE question often asked, "What kiµd of negative is required
for carbon printing?" implies that some negatives are
Th.ese n~ks may be guarded against by the use .of ice or
unsuitable for the process, this, however, is a suggestion
free~1!3g mixtures to cool the sern~itizing bath, and by the
I am riot disposed to admit. The two classes of negatives
~dd1tion of a proportion of spirit to the solution, thus minimis
likely to present difficulty in printing are the flat thin negative
m& the s~ftening due to a wh~lly aqueous solution, aiding
with ·veiled shadows, and the negative which is dense in
9mck. dr¥1~g and_ raising very considerably the temperature
addition to being underexposed; apart from extreme instances
m which it is P?ss1ble to dry the tissue without risk of melting.
in ·these_t wo classes (which na_turally will not yield the range
The proportion of alcohol added may vary according to
of gradation obtainable with a good negative) any negative
the temperature from rn to 25 per cent. of the bulk of the
sensitiz~ng bath. will give a go()d carbon print, discretion having been exercised
.If the at~osphere is extr~mely humid, delaying drying, it in th~ s_trength of the sensitizing bath employed as indicated
in the section devoted to sensitizing.
will ~e advisable t~ add sufficient liq-gid ammonia to the c.
The negative must be provided with, what is known as
solution to change it from its reddish colour to a decided
a" Safe Edge,'' that is, a narrow margin of the negative must
yel~ow, the ammonia serving to counteract the insolubilizing
be rendered opaque either by strips of yellow paper or black
action of a prolonged period of drying.
Methylated alcohol may usually be· employed, but it is va~nish, applied preferably on the back of the negative. The
?- somewhat variable preparation, therefore rectified alcohol object of the safe edge being to ensure a soluble margin to
is preferable where the price is not excessive. the ~issue by protecting it from the action of light; this
soluble margin preventing any risk of the picture washing up
~ sensitizing bath containing alcohol deteriorates more
at the edges during development.
qmckly than an aqueous solution, and must be kept in the dark.
Gen~ral. Waterhouse strongly recommends for tropical ~hould, however, it be desired to develop on rough or
thick paper, with a clean margin thus avoiding the need .of
countnes m the the rainy season the use of a calcium chloride
drying box. ' m?unting the print, tl:ie safe edge may be put on the front
15
. 14
of the negative, thus giving a sharper delimination to the eating insufficient exposure, over exposure on the contrary,.
print. giving too dark a print.
As it ~s i~port~nt to keep the tissue d~y while in the printing I am w~ll aware that there exists a prejudice against the
frame, it is advisable to ·use a piece of American· cloth or use of actinometers. I do not use the term prejudice in any
other non-absorbent material between the tissue and the pads. offensive sense, but merely as indicating an opinion formed
of the printing frame. before trial, for I am convinced that in practice the use of
Dry tissue having a' t~nden,_cy- to curl. may be found some an actinometer is easy, convenient, and reliable-for as soon
what .awkward. to get ~nto. contact with the negative, but as by experiment the correct exposure, that is, the number of
war~mg th~ piece of Amencan cloth before putting it on to tints required for one particular negative, has been determined,
~he hss1:e will be found helpful in getting the tissue flat and a record has been established and the negative for which the
m .en~urmg perfect contact; the backing cloth or pads in the correct exposure has been settled serves as a criterion and
prmtmg frame should be somewhat thicker than those needed ready means of comparison with other negatives of varying
with silver paper as the tissue being less pliable than silver densities. ·
pape: n.eeds more pressure t.o ensure complete contact. The tint on the Disc actinometer is comparatively light,.
Prmtmg should be done m the shade the heat of direct quickly printing to the standard tint, the actinometer, there
sunlight cracking the tissue. ' fore, requires constant watching to turn the lid when the
will
Carbon tissue sensitized in a 4 per cent, bath be found tint is arrived at, but the delicacy and accuracy of the tint
to be about _three ~i~es. as sensitive as albumenized· paper> renders this actinometer particularly suitable for thin negatives~
but as there is no visible image an actinometer will be needed Another slightly more costly but more convenient actino
to gauge the exposure. meter is known as the "Johnson," in form a cubical box,
Various form~ of actinometers are made. The simplest containing a roll of sensitive paper which is pulled forward
an~ cheapest bemg that known, from its form, as the" Disc'~ under the lid as each tint is registered; the principle on which
actu:~~meter. It consists of a box, containing a disc of it works is identical with that of the Disc.
sensitive paper, and provided with a: movable lid. On the Both these actinometers give what may be called an
lid is a reddish brown segment of a circle in tlie centre of int~rmittent record of tints requiring that the lid should be
~hich is .a small round transparent spot through which the partlyrotated in the case of the Disc, that the strip of sensitive
hght reaches t~e sensiti~e paper. On exposure to light the I paper should be p.illed forward in the case of the Johnson
paper under this spot will gradually darken untir it assm~es as soon as each tint is printed.
the sam~ hue or'tint ~s the surrounding part of the segment The " Sawyer " actinometer is also constructed on the
of the circle, when this occurs it is considered that a " tint principle of tints, but in its case the record of tints is not
H
or unit of exposure is registered, and the lid is slightli/otated intermittent and dependent on the sensitive paper being
to expose a. fresh part of the paper, the operation'tbeing drawn forward, but is continuous, the lid of the actinometer
repeated until the desired number of tints is recorded. ~'.·:~ ~aving a graduated scale or series of tints bearing numbers.
With a beginner the exposure needed for a carbon print m c~e~r glass from 1 to 9. Under the screen is a strip of
m~st, of course, in the first instance be experimental (regaxd sensitized p~per having on either side of it the standard tint.
bemg had to the relative sensitiveness between silver and On exposure to light the ground surrounding each number
carJ:>on as previously mentioned) the result not being seen darkens in succession to the depth of the standard tint,
until development, which operation tells at once whether co~sequently no moving of the paper is needed, the instrument
exposure has been correctly° judged-too light a print indi- bemg self-recording to the extent of 9 tints. ·
