Charles Tomlinson.

Cyclopædia of useful arts, mechanical and chemical, manufactures, mining, and engineering (Volume 2) online

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and animals ; scarcely an attempt to show the action
of the actinic spectrum on chemical preparations, or
on natural colours ; no impressions of the lines in the
photographic, corresponding to those in the luminous
spectrum ; no copies of pages of ancient manuscripts ;
no miniatures of printed books, &c. It is well re-
marked in the Jury Report, that " photography holds
a place at present intermediate between an art and a
science, a position eminently favourable to develop-
ment in either direction. Its pursuit as an elegant
and most extensively useful art, affords a strong mo-
tive for inquiry and experiment in the improvement
of its processes ; in the course of which, an infinity
of facts, new and unexpected, come forward, every
one of which may turn out to be the embodiment of
some pregnant scientific principle; nay, even the
smallest minutiae of manipulation, on which it is
found that success or failure in the production of
artistic effect depends, may, if duly observed and
reasoned on, afford indications, linking together the
known and the unknown in optical science, and tend-
ing to bring these mysterious operations of light
within the pale of exact reasoning. On the other
hand, science is too much in the habit of repaying to
art, with interest, every assistance of that nature, to
leave room for doubt of similar results in this in-
stance, when once the principles of operative che-
mistry shall have assumed a definite form and sub-
jective connexion. It is this which affords us full
assurance that photography is yet in its infancy, and
that all which has been hitherto accomplished
marvellous and exquisite though it be is as nothing
to what will be performed when the veil shall be
removed, which, for the present, obscures its true
scientific principles." The rapid advance of the art
during the brief period of its existence, is well
illustrated by the fact, that the method formerly
adopted for procuring daguerreotype portraits re-
quired a person to sit without moving for 25 minutes,
in a glaring light, whereas, at the present day, the
effect is produced almost instantaneously.

The introduction of the accelerating process by
M. Claudet, at once improved the practice of por-
trait-taking ; two daguerreotype establishments were
formed in London, and although the portraits taken
were deficient in expression and had other defects,
yet the receipts at these establishments several times
amounted to 60 in one day. The use of Mr. Talbot's
Calotype process for portraits, by Mr. Collen, was a
further improvement in this branch of the art ; and
as the effect of the picture could be heightened by
the brush, defects of expression could be removed,
and the likeness improved, at subsequent sittings.

During a portion of the months of December 1852
and January 1853, the Society of Arts, London, held



an exhibition of photographs, collected from the most
distinguished artists and amateurs in this country,
and some from continental artists. The specimens
exhibited exceeded 1,000 in number, and afforded a
very favourable idea of the condition of the art at the
time. On the 26th January, Mr. Glaisher read a
paper to the Society, " On the chief Points of Ex-
cellence in the different Processes of Photography, as
illustrated by the present Exhibition." The follow-
ing observations are abridged from the printed report
of this lecture. Of the various photographic pro-
cesses now in use the range of practice assigned to
the calotype has been very general, with a leaning,
however, to outdoor and local scenery. That of the
wax-paper has been more strictly defined ; and on the
Continent we find it employed in architectural designs,
and fragments of carved and massive ornamentation.
In England it has been chiefly employed to perpetuate
the passing scene, with little discrimination as to its
character. The albuminised glass has been applied to
general representation, such as views, landscapes and
groups of statuary. The albuminised paper has also
been employed upon groups of statuary. The col-
lodion has furnished designs of various character,
including the whole of the portraiture in this exhibi-
tion. Some of the effects of the collodion process
are so admirable, and the points of failure in all so
much less exaggerated than those either of paper or
albuminised glass, that this appears to be the process
which ought to be specially cultivated : it generally
exhibits a natural interpretation of the lights and
shadows, which rarely fails to communicate a similar
effect to the subject. In fact, this process is less
eminent in failure, and more eminent in success, than
the other processes illustrated in the exhibition, for
it combines the excellencies of our best photographs,
with fewer of their defects.

The difficulties of representing woodland and forest
scenery were evident in this exhibition : the tree,
whether alone and filling the central area of the
picture, or one of several, is more imperfectly repre-
sented than any of the many creations of art and
industry. Very few of the trees are perfect in defini-
tion towards the top and outer branches, arising from
their continued stirring with the motion of the air.
In the same manner the gentle movements of the
leaves in summer tend to produce confused results.
A more sensitive medium than the paper is required,
upon which to obtain an instantaneous impression.
This want may probably be supplied by the collodion,
or by Mr. Talbot's instantaneous process. 1

The representation of running water is a difficulty
that points also to a highly sensitive process, and an



(1) It appears from recent experiments, that the exposure of
the prepared surface to the optical image for an instant is suffi-
cient to produce a perfect photographic picture. A printed paper
was attached to the face of a wheel, and this was made to rotate.
The camera with the prepared photographic surface was placed
opposite to the wheel and properly adjusted, and then the room
was darkened. The wheel was next illuminated for an instant by
a strong spark from the conductor of a powerful electric machine,
and this instantaneous appearance of the wheel before the camera
was sufficient to produce a perfect picture.



PHOTOGRAPHYPHOTOMETRY.



405



instantaneous impression. In a water-mill on paper,
the water descending in a body from the trough above
the wheel gives the idea of a soft and rounded mass,
the apparent rotundity being conveyed by the shadow
which rounds the edge ; the characteristics of water
as exhibited under any circumstances arc totally lost.
In short, quickly running and ruffled water in the
present collection has not been in any case depicted
with satisfactory results.

Paper appears to be well applied to subjects of
no great finish and delicacy, and its general tone
is to be preferred to that of wax-paper. The latter
is most frequently recognisable by great strength of
tone and by the prevalence of a citrine hue, which is
very objectionable in excess. This process seems to
exceed the paper in the power of discriminating
material, and some of the finest specimens in the
collection are due to its employment.

The glass processes, either by albumen or collodion,
appear to be best fitted for conveying subjects of a
smooth and delicate nature. The specimens exhibited
showed the greatest finish and delicacy, and it is
thought that the collodion will supersede the albu-
minised glass.

Many of the photographs are wanting in vertically,
in consequence of not properly adjusting the visual
axis of the camera. The camera ought to be furnished
with a spirit-level to secure horizontal adjustment in
the field with facility and certainty. In some cases
indifferent object-glasses appear to have been used,
and in some others, where the object-glass has been
good, it has been so ground as to give good definition
within very narrow limits.

" Whether photography will ever exist as an inde-
pendent art, without assistance borrowed from the
artist, is a matter of pure speculation. At the pre-
sent time there is much to be done before this most
graphic process can approach within even near limits
to the beautiful semblances of nature which are
preserved in the works of our best artists. It is
necessary that the photographer should receive a
better artistic education; that he should be better
acquainted with those laws belonging to science by
which the canvass is made to assume the semblance
of some of nature's most agreeable effects: it is
necessary that he know how to choose his point of
view ; to decide upon the proper balance of light and
shade ; to have a correct appreciation of the strength
of outline and development of parts belonging to the
distances of his picture ; that he should not resort to
violent contrasts for effect, and that he should choose
that tone most in accordance with his subject. The
true knowledge of these, among other things, must
belong to the photographer who would step beyond
the level of ordinary practice. To the artistic spirit
infused into the Photographic Society, so newly
organised, we must look for his better guidance in
reference to those points of study ; but with all its
imperfections, photography may be considered as
sufficiently under control to be rendered a subsidiary
and highly useful art." In the present collection there
or*. inrlinntirmci nfit.ftHTm1ina.tion to the microscoDC. and



to the .medical profession in the portraits of persons
afflicted with mental disease; admirable copies of
engravings were also shown, and a few illustrations
of tropical scenery.

In conclusion, we may just refer to the application
of Mr. Wheatstone's beautiful instrument, the stereo-
scope, to photography ; to the attempt to transfer the
collodion picture from the glass to the wood en-
graver's block ; and to the success which has long
attended the use of sensitive paper in furnishing a
sort of perpetual register of meteorological and mag-
netic instruments.

PHOTOMETRY is the art of measuring the
relative intensities of different artificial lights. The
variations in the intensity of light cannot be measured
by an instrument in a manner similar to that by which
variations in temperature are measured by the ther-
mometer, or in the pressure of the atmosphere by the
barometer. Although the quantity of light which
falls upon the earth differs greatly at different times,
depending upon the height of the sun above the
horizon, upon the presence or absence of clouds, and
the state of the atmosphere, yet we have no means of
comparing days together with respect to their light,
as we are accustomed to do as regards their heat.
We have no instrument or contrivance by which light
alone can be made to produce mechanical motion, so
as to mark a point on a scale, or to give a direct
reading off of its intensity or quantity at any moment.
" This obliges us to refer all our estimations of the
degrees of brightness at once to our organs of vision,
and to judge of their amount by the impression they
produce immediately on our sense of sight. But the
eye, though sensible to an astonishing range of dif-
ferent degrees of illumination, is, partly on that very
account, but little capable of judging of their relative
strength, or even of recognising their identity, when
presented at intervals of time, especially at distant
intervals. In this manner the judgment of the eye
is as little to be depended on for a measure of light
as that of the hand would be for the weight of a body
casually presented. This uncertainty, too, is increased
by the nature of the organ itself, which is in a con-
stant state of fluctuation: the opening of the pupil,
which admits the light, being continually contracting
and expanding by the stimulus of the light itself, and
the sensibility of the nerves, which feel the impression
varying at every instant. Let any one call to mind
the blinding and overpowering effect of a flash of
lightning in a dark night, compared with the sensation
an equally vivid flash produces in full day-light. In
the one case the eye is painfully affected, and the
violent agitation into which the nerves of the retina
arc thrown, is sensible for many seconds afterwards,
in a series of imaginary alternations of light and
darkness. By day, no such effect is produced, and we
trace the course of the flash, and the y.ig-zags of its
motion, with perfect distinctness and tranquillity, and
without any of those ideas of overpowering intensities
which previous and subsequent total darkness attach
to it. But yet more. When two unequally illumi-
nated objects (surface of white paper, for instance)



406



PHOTOMETRY.



are presented at once to the sight, though we pro-
nounce immediately on the existence of a difference,
and see that one is brighter than the other, we are
quite unable to say what is the proportion between
them. Illuminate half a sheet of paper by the light
of one candle, and the other half by that of several,
the difference will be evident. But if ten different
persons are desired, from their appearance only, to
guess at the number of candles shining on each, the
probability is that no two will agree. Nay, even the
same person, at different times, will form different
judgments. This throws additional difficulty in the
way of photometrical estimations, and would seem to
render this one of the most delicate and difficult
departments of optics." l The eye is, however, able to
judge with tolerable accuracy of the equality of two
lights, or of two degrees of illumination seen at once.
Thus, in order to ascertain the relative quantities of
light furnished by two different lamps, place two
discs of white paper a few feet apart on a wall,
and throw the light of one lamp upon one disc, and
the light of the other lamp upon the other disc : if
the lamps are of unequal illuminating power, the
lamp whicli affords most light must be moved back
until the two discs are equally illuminated. Then on
measuring the distance between each lamp and the
disc which it illumines, the luminous intensities of
the two lamps may be calculated. As the intensity
of light from a point or luminous source diminishes
inversely with the square of the distance, [see
LIGHT. See also HEAT, Pig. 1135,] so the intensities
of the two lamps are to each other as the square of
the distance. If, when the discs are equally illumi-
nated, the distance from one lamp to its disc is double
the distance of the other lamp from its disc, then the
first lamp is 4 times more luminous than the second ;
if the distance Vje triple, it is 9 times more luminous,
and so on.

In this experiment the eye cannot judge very accu-
rately as to the exact degree of equality in the light
thrown upon the two discs; but by means of two
conical tubes, Eig. 1616, united at their smaller ex-




Fig. 1616.

tremities, and terminating there in two discs of paper,
it is more easy to decide when the discs are equally
luminous. If, for example, it is desired to compare
the light of a candle with that of a lamp, in order to
ascertain how many candles will give a light equal to
that of the lamp, the two lights are placed so far
apart that their rays shall not interfere, the broad end
of one of the tubes being directed to the lamp, and

(1) Herschel, Encyclop&dia Metropolitana, article LIGHT.



that of the other tube to the candle. The lamp is
drawn back, or the candle moved forward, until the
two discs are equally luminous. The distances are
then measured as before.

A still better arrangement than the above was
contrived by Professor Ritchie, and is shown in
section, Eig. 1617. It consists of a rectangular box,
open at both ends, and blackened within to absorb




Fig. 1617.

extraneous light. At the top is a long narrow rect-
angular slit, A B, covered with tissue or oiled paper.
Within are two pieces of looking-glass, c D and c E,
cut from the same piece, in order to secure uniformity
of reflection. Each mirror is of the width of the box,
and its reflecting surface is turned towards the open
end of the box. The upper edges of the mirrors meet
at c, and the line of junction divides the space A B
into two equal parts : it is moreover covered with a
piece of black card, to prevent the mingling of the
lights reflected from the two mirrors. In using this
photometer, as it is called, it is placed between the
lights, the intensities of which are to be compared, so
that they may be reflected from c D and c E, upon
the tissue paper AB. The instrument is then brought
nearer to one or other of the two lights, until, to an
eye placed above A B, the two portions A c, B c, ap-
pear to be equally illuminated, which may be judged
of with tolerable accuracy. The distances must be
measured from the vertical c P. In viewing the illu-
minated surface A B, the eye should be protected
from extraneous lights; for which purpose a prismatic
box is provided; it is about eight inches long, and
blackened within to absorb shining light. One end of
this box is to rest on the illuminated surface A B, and
the other is to be applied close to the eye. Instead
of using the mirrors and the paper screen AB, the
inclined planes, c D, c E, may be covered with while
paper, and viewed directly through the aperture.
But however the instrument be used, a mean of
several observations should be taken, ths box being-
turned round after each. When the lights compared
are of different colours, as daylight, moonlight, and
candlelight, the space A B is to be covered with a
piece of fine white paper, printed distinctly in a
small type ; the paper is to be brushed over with oil,
and the box being placed between the lights, it is to
be moved until the printing can be read continuously
along the paper, with equal ease on both sides of the
line c. Or the printed paper may be pasted on the
mirrors, or the inclined surfaces on which they rest,
and the print is then to be read through the opening,
which may be enlarged for this method of applying
the box.
Professor Wheatstone has constructed a photo-



PHOTOMETRY.



407



meter of greater accuracy and convenience than any
yet contrived. It depends for its action on the per-
manence of the impression of light upon the optic
nerve, and is thus described by Professor Danicll :
"If a small convex reflector, such as a bead of glass,
about th inch in diameter, silvered on the inside,
be placed between two lights, bright images of both
will be formed, but differing in brightness according
to the intensity of each. A rough estimate of their
relative values may be obtained by adjusting the dis-
tances between the two ; but by causing the bead to
move backwards and forwards in a straight line, two
parallel lines of light will be formed, about T Vth
inch apart, instead of two spots. By moving the
reflector to different parts of the line which joins the
two lights, or by changing the relative distances of
the lights themselves, these two luminous lines may be
made to appear perfectly equal in brightness. The com-
parative value of the lights may then be ascertained
by squaring the distances." 1 In Eig. 1618 the instru-




Fig. 1618.

ment is represented of the full size, c c' c" is a circle of
brass fixed to the wooden frame r ; the inside of the
brass circle is toothed, so that a smaller circle, c c",
half the diameter of the larger one, and toothed at its
circumference, may engage the teeth of the larger
circle. The small circle turns upon its centre c', and
is fixed to the arm a c', which is moveable upon the
centre a of the large circle by means of a key on the
opposite side of the frame. The bright metallic bead
b is attached to the circumference of the small circle,
and as this circle is carried round by the arm a c', it
also rotates rapidly upon its centre c', and the bead
travels along the diameter of the large circle from b
( o c in completing one half of its revolution, and back
again from c to b during the other half.

Count Rumford devised a method of comparing the
intensities of two lights by means of the shadows
which they respectively cast. The two lights are to
be so arranged that each may cast upon a plane white
surface a shadow of a small object, such as that of
a book or an upright rod, as in Fig. 1619 : the eye
can form a tolerable judgment as to the relative dark-



ness of these shadows. The brighter light, which casts
the deeper shadow, is to be removed, or the weaker
light brought nearer, until the two shadows are equal-
The distance of the two lights from the object



ized.




Fig. 1619.

which intercepts their rays being measured, the relative
intensity of the lights will be, as before, inversely as
the squares of the distances. The shadow of one
light is illuminated solely by the rays of the other,
while the surrounding space is illuminated by the rays
of both ; when, therefore, the shadows are equal, the
lights are equal. Count Rumford applied this method
to measure the variations in light, by a candle left
unsnuffed. Supposing the light furnished by a pro-
perly snuffed candle to equal 100, and it be allowed
to burn for 11 minutes, the light will then be equal
to only 39, and if allowed to burn without snuffing
for 30 minutes, it will only be equal to 16.

Professor Leslie's photometer is constructed on
the assumed principle that light is convertible into
heat. It consists of the differential thermometer, of
which one of the bulbs is blackened and the other
clear: a glass case is put over the instrument to
exclude rays of heat. In order to use it, the light to
which it is exposed passes through the clear ball, but
is absorbed by the black, and supposing this light to
be converted into heat, the rise of temperature will
indicate the degree of illumination. The instrument
is certainly not correct in principle; it is in fact
nothing more than a delicate air thermometer.

A simple and ingenious photometer in use at the
"Westminster Gas-works, consists of a disc of paper,
4 or 5 inches in diameter, the outer portion of which
is waxed, and the inner portion left plain, so that on
holding the paper vertically between the eye and a
luminous flame, the waxed portion is seen in the form
of a translucent ring, while the unwaxed portion
appears as an opaque disc. The disc is mounted ver-
tically upon a short cylinder of wood, sliding in the
groove of a horizontal frame, at the extremities of
which are fixed the lights to be compared. This
frame also contains a scale graduated into feet and
inches. The sliding piece which carries the disc is
also furnished with a pointer, so that the distance of
the disc from either flame can always be ascertained
at a glance. In using this apparatus, the observer
stands on one side in such a position as to get the
disc fairly between his eve and one of the flames.
The waxed ring is then illuminated by the transmitted
light of the concealed flame, while the reflected light
oF the front flame illuminates the opaque disc. The
nnnnr is then moved to and fro until the transmitted



408



PIANO-FORTE.



light and the reflected light appear to be equal.
When this is seen to be the case on one side, the
observer passes to the other, where the conditions are
reversed, but the result is the same ; the light which
on the other side lighted up the opaque disc, now
illumines the waxed ring, and vice versa. The dis-
tance of the paper from either light is then measured,
and the power of each flame is calculated as before.

PIANO-FORTE. This well-known instrument
belongs to that class of stringed instruments in which
the sounds are produced by imparting vibration to
elastic strings extended tightly over a case or box,
and covered with thin boards, the vibrations of which,
imparted to the volume of air which they enclose,
assist the development of sound.

It is remarkable that while poetry, architecture,
sculpture, and probably painting, attained their high-
est state of perfection among the ancients, it has been
reserved to the moderns to achieve excellence in music ;
and the cause is closely connected with the superior
mechanical skill of the moderns. If the progress of in-
strumental music depend upon the perfection of the
mechanism of the instrument which gives it voice and
meaning, music, as a fine art, must be in a more ad-
vanced state now than at any other period, since our
mechanical resources were never at any former period
so great, or so fully developed. The music of the old
composers exhibits, in a very marked degree, the
capabilities of the instruments for which it was written,
and these were so limited, that to attempt to perform



Online LibraryCharles TomlinsonCyclopædia of useful arts, mechanical and chemical, manufactures, mining, and engineering (Volume 2) → online text (page 95 of 245)