IMPROVEMENT OF PHOTOMETERS FOR ESTIMATING PHOTO- GRAPHIC EXPOSURES BY THE EMPLOYMENT OF STANDARD PARLIAMENTARY CANDLES
Year: 1885

page_text: 400 шрготошень OI гnotometers for Estimating Photographic
Exposures by the Employment of Standard Parliamentary
Candles.
I, HAROLD DENNIS TAYLOR of 6, Grosvenor Terrace, Bootham, York, in the
County of York, Astronomical Instrument Maker, do hereby declare the nature of
my said invention for the IMPROVEMENT OF PHOTOMETERS FOR ESTIMATING PHOTO-
GRAPHIC EXPOSURES BY THE EMPLOYMENT OF STANDARD PARLIAMENTARY CANDLES,
to be as follows:-
The object of my invention is to supply the photographer with a new and
reliable means of knowing how long he must expose his sensitive plate or film to
the action of the luminous image of the subject thrown upon it by the lens, he,
in the first place having ascertained the degree of sensitiveness of his plate &
knowing the rapidity of his lens. In its construction my invention consists
essentially of the following parts.
1st. A light-tight lamp in which a Standard Parliamentary Candle is enclosed
as a luminous source. This lamp is so constructed that the air necessary to the
combustion of the candle finds its way into and out of the lamp by a devious
course, through which the exterior day-light cannot penetrate into the lamp in any
hurtful degree. Thus, when the lamp is lighted, the only light inside the lamp is
that supplied by the Standard Candle. I have selected the Standard Parliamentary
Candle as the source of light to be used in the manner & for the purposes described
below in this specification, as it is eminently portable and can be relied upon as
affording a light which is constant enough for the purpose aimed at in my invention.
Opposite to the centre of luminosity of the candle-flame & fixed to the side of the
laap is a short horizontal piece of tube, its axis pointing to the centre of the
candle-flame. The side of the lamp is cut away within the junction of the side
tube so that the light from the candle may shine upon and illuminate a disc of
opal glass or thin porcelain which will then appear as an evenly illuminated white
disc. This opal or porcelain disc is fixed just within the short piece of tube near
its junction with the lamp. This porcelain or opal-glass disc, being fixed at a
constant distance from & illuminated by the candle-Hame, forms an illuminated
surface of a brilliancy as constant as that of the candle-flame & it is evident that,
under the above conditions, the light thrown out by the white disc will depend
ED
077

page_text: upon and vary with its area. As close as possible in front of the illuminated disc
& sliding or rotating through slits in the tube is a metal strip with a graduated
series of apertures cut out in it, all in a straight line or round a circle, as the case
may be. Small nicks are cut in the edge of this metal strip or plate (which I shall
call the diaphragm-slide) into which a spring engages for the purpose of just
holding the diaphragm-slide whenever one of its apertures comes centrally over the
illuminated disc. These apertures limit the area of the illuminated disc to their
own size or area. They may be square, but I prefer them circular.
2nd. A small tube (which I shall call the observing tube) with an aperture cut
in its side has another short piece of tube or hollow flange affixed to it so that the
axes of the two are at right-angles. The latter short piece of tube or hollow flange
fits rather stiffly within the tube holding the illuminated disc & diaphragm-slide,
which latter tube thus connects the lamp & observing tube together. The light
from the illuminated disc thus shines along the short connecting tube, through the
above mentioned aperture in the side of the observing tube, into the observing
tube itself, where it strikes upon a thin diaphragm placed diagonally within the
latter. This diagonal diaphragm has a small hole cut in its centre & immediately
round this hole the diaphragm is painted or otherwise coloured a pure white.
In that end of the observing tube from which the white ring on the diagonal
diaphragm is visible, is placed a lens (somewhere about 2 or 3 inches from the
diagonal diaphragm). This lens is of such a focal length as will enable any person
of average eyesight to see the hole in the diagonal diaphragm distinctly. On that
side of the diagonal diaphragm opposite to the lens, the observing tube is continued
for perhaps an inch & ended or stopped by a disc with a hole in it a little larger
than the hole in the diagonal diaphragm. This is merely to stop out stray light as
much as possible.
The function of the lens in the observing tube is twofold; 1st. to enable the hole
in the diagonal diaphragm to be seen with well defined edges & 2nd. but most
important to so throw out of focus & blur all details seen through & beyond the
hole in the diagonal diaphragm as to render the light received through the hole
from such details visible in the form of an almost uniform disc of light bounded
by the edges of the hole. Such a spot of uniform light is much more adapted for
comparison with the lamp illuminated white ring surrounding it, than a small
portion of the subject to be photographed shewing all detail, such as would be
visible through the hole to a normal eye, if no lens were used.
Fixed behind or in front of the lens is an absorbent medium of a transparent
blue colour, whose function it is to only allow those rays of light to pass through
it, to which an ordinary photographic plate or film is sensitive. These are almost
wholly the blue-green, blue & violet rays of light. I cannot find any specimen of
blue glass, which, by itself, will let only these rays pass through, but I can isolate
these rays by using two sorts of coloured glass combined, one of them, the ordinary
cobalt blue glass & the other of a peacock-green colour. Thus both the light
received through the hole in the diagonal diaphragm and the candle light reflected
from the white ring surrounding it, is viewed through the glass & appears of an
uniform blue colour, there being little room for any discrepancy of colour between
the light received from the two sources, thus rendering comparison as easy as
possible.
The instrument is used as follows:-
In the first place a few experiments must be made for each brand of plates, as
the different brands vary in rapidity. A few varying exposures are made for some
particular portion of a subject, the object being to get a plate so exposed that that
particular portion shall only be just indicated on the plate (after normal develop-
ment) in a perceptible degree, that is, the details of the selected portion are to be
just indicated in the resulting negative. At the time of the trial exposures, the
photometer lamp is lighted and, after being allowed to burn more than one minute,
the lamp is held upright & the observing tube is directed towards the above
mentioned selected portion of the subject. The observing tube can be directed up

page_text: or down with facility owing to its connection with the tube carrying the illuminated
disc and diaphragm slide. The latter is drawn in or out or rotated until the light
from the white ring seems of equal intensity with the light from the subject seen
through the hole, & the aperture of the diaphragm slide which is then over the
illuminated disc is indicated by a corresponding number marked on the slide. The
apertures are numbered in inverse ratio to their areas, thus, the largest is No. 1 &
an aperture half its area is numbered 2 & so on, these numbers indicating relative
exposures required. As soon as a negative has been obtained which just faintly
indicates detail at that part of the image corresponding to the portion of the subject
from which the photometer reading was taken; the exposure that this negative has
had in the camera, say 6 seconds, is divided by the number which was indicated by
the photometer, which may have been (say) 8. The result is sec. that is, the
exposure which would have been required just to imprint the same detail if the
photometer had registered one. This quantity, (of a second in the above instance)
is what I call the "constant" for that particular brand of plates. If in actual
practise, the photometer be used for estimating the exposure for any particular
subject using the same brand of plates, the instrument is directed towards the
darkest portion of the subject where the eye can recognise detail, the photometer
reading is taken and multiplied by the "constant ', or a table prepared with the
multiplications already worked out may be referred to.
If the same stop is used in his photographic lens as that used in the above trial
exposures, then he will have, by the above multiplication, the exposure necessary
to just indicate on the plate the detail in that dark part of the subject of which he
took an observation. If not, the relative area of the stop he is using to that one
used in the trial exposure must be known and allowed for, as has to be done by the
photographer in any case.
If the photographer will call the largest stop of his lens No. 1 & number the
others in inverse ratio to their areas as is done in the case of the apertures of the
diaphragm slide of my photometer, then in obtaining his "constant" he must, after
obtaining the right trial exposure, divide the latter not only by the number read
off from the photometer but also by the number of the stop used in the lens. Then,
in estimating exposures he will have to multiply the number indicated by the
photometer by the "coustant" & that again by the number of the stop he is going
to use in his lens. The " constant," in the above method of procedure, is that
exposure necessary to just indicate detail on the plate when No. 1 stop is used in
the lens during exposure & when, at the same time, the photometer (being directed
to that detail) gives a reading of one or unity. Since after taking a reading, the
photographer knows that a certain exposure will give a certain result on the plate,
then it is evident that he has a certain basis for his judgment to work upon & may
give more or less exposure if he likes, the instrument not interfereing, in any degree,
with the individual taste of the photographer To "expose for the shadows &
let the high-lights take care of themselves" is, for most subjects, the principle
acknowledged by the photographer & my photometer is designed to give results in
accordance with that rule.

page_text: The Improvement of Photometers for Estimating Photographic
Exposures by the Employment of Standard Parliamentary
Candles.
I, HAROLD DENNIS TAYLOR of No. 6, Grosvenor Terrace, Bootham, York, in the
county of York, Astronomical Instrument Maker do hereby declare the nature of
my invention for THE IMPROVEMENT OF PHOTOMETERS FOR ESTIMATING PHOTO-
GRAPHIC EXPOSURES and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:-
The
In the accompanying diagrams, Fig. 1. is a side elevation of the instrument, con-
sisting of the lamp L-L-C & the observing tube T-T which is shown turned
down parallel to the lamp. Fig. 2. represents the instrument as seen if viewed
from the left in Fig. 1. and with the observing tube T-T¹ removed. Fig. 3. is a
vertical section of the instrument showing interior construction & T-T¹ is shewn in
cross section. Fig. 4. is a sectional plan of the instrument with the tube T-T¹
shewn in longitudinal section. The same parts are similarily lettered throughout.
The lamp L-L-C is composed of two separate parts, one within the other; the
inner part being composed of the candle tube or holder C-C and the inner casing
of the lamp (see Figs. 3 & 4), the two portions being united together by the
perforated ring shaped plate shewn in section in Fig. 3 & in plan in Fig. 4.
inner casing of the lamp rotates stiffly within the outer casing and has an aperture
cut out of one side as shewn in Figs. 3 & 4, where it lies opposite to the aperture
in the outer casing. The inner casing is rotated until its aperture coincides with
the open aperture A in the outer casing, whenever the candle has to be lighted or
extinguished. The chimney of the lamp and the short branch tube B are fixed to
the outer casing of the lamp. The lamp is, as much as necessary, rendered light
tight, as the air supplying the flame passes in round the inner edge of the flange or
cap P-P, then round the outer edge of the thin flange just above it & thence
through the holes in the perforated plate above mentioned. Light is prevented
from directly entering the lamp through the holes in the chimney, by means of the
cap enclosing them. In Figs. 3 & 4. O is a disc of opal glass or porcelain. Sliding
through B & close in front of O is the metal slide S-S carrying a graduated
series of apertures, which are numbered in inverse ratio to their areas. This slide
has notches cut in one edge at distances corresponding to the distances separating
the apertures. A spring 5 p, engages one of these notches whenever one of the
apertures comes centrally over the disc O, and the number of such aperture is
marked close by its corresponding notch. K is a hollow tube or socket rotating
stiffly within B and is connected by a short piece of tube with the observing
tube TT fixed with its axis at right-angles to the axis of B or K. Thus the
tube T-T can be pointed in any direction up or down. At the intersection of
the axes of the tubes B & T-T¹ is placed a diaphragm D of thin metal with a
sinall hole cut in its centre. Immediately round the hole the diaphragm is painted
with a ring of good white pigment. At the end T of the tube T-T is a disc of
glass, which is selected with a view to only allowing those rays of light to pass
through it, to which the ordinary photographic plate is sensitive. In the case of

page_text: the photographic sensitive plates mostly in use, it is the greenish-blue, blue and
violet rays that exercise the chemical effect, and I find that the medium which only
allows these rays to pass through it and absorbs the others, is the ordinary cobalt
blue glass combined with a certain peacock green glass. The other end T¹ of the
tube carries an eyepiece containing a lens (l) of such a focal length as will enable
any average sighted person to see the hole in the diaphragm D with distinctness.
In my provisional specification I described the coloured glass as being placed next
to the lens, but as this method was found, on experiment, to render the white ring
on the diaphragm D too dim to be easily seen, I now place it at the other end T of
the tube, so that the white ring is seen undimmed and only that light coming
through the hole in D has to pass through the coloured glass.
Behind the lens (2) in the eyepiece is a small disc of ferrotype or other thin metal,
with a V shaped, tapering aperture cut out in it.
Its exact shape is shewn in Fig. 5, where it is represented by black.
If no lens were placed in the tube T-T¹ and the tube were directed towards any
object and the eye placed at T¹, then the hole in D would be too close to the eye
to be seen distinctly, its outline would appear blurred and the details of the object,
towards which the tube was directed, would be distinctly visible. The object of
the lens is to reverse these conditions; viz. to make the outline of the hole in D
appear distinct and to render the distant details completely blurred, that is, to
reduce them to a sufficiently uniform patch of light.
The principal and use of the instrument is as follows:-
In the first place, when the candle is burning and the lamp held vertical, the
opal glass disc O is illuminated and yields light of an intensity as uniform as that
of the Standard candle. The light from the disc O falls upon and illuminates the
wbite ring on the diaphragm D, while the amount of light from O is regulated by
the apertures in the slide S-S. These apertures are shewn in the diagrams of
such relative diameters that their areas are in inverse ratio to their corresponding
numbers marked by the notches in S-S. Thus, if aperture 200 is centrally over O,
then the white ring on D is illuminated with 100 of the intensity with which it
would be illuminated, were aperture No. 1 centrally over O.
and the
In using the instrument, the candle is lighted and allowed to burn up
lamp closed. The lamp is held upright in one hand and the observing tube is
directed to (as a rule) the darkest part of the subject to be photographed. The
hole in the diaphragm D then appears to the eye placed at T as a spot of light
(as a rule blue), bright or din according to the brightness of the shadow to which
the observing tube is directed. The slide S-S is pushed up or down until the
white ring appears of about the same brightness as the spot of light in its centre,
and the number of the aperture in S-S then in use is read off. The apertures in S-S
are numbered in inverse ratio to their areas because these numbers then directly
represent the exposure required. If a certain shadow of a subject yields a reading
of 1, it is evident that another shadow giving a reading of 30, requires 30 times
the exposure in the camera that the former does.
The wedge-shaped hole in the eye-end T¹ of the observing tube is used for the
following purpose.
If the light coming through the hole in D is of such a brightness that the
reading of the photometer lies between 1 & 8 or so, it is found that there is
difficulty in judging when the white ring is equal in brightness to that of the
small spot, and this difficulty is chiefly owing to their difference of colour. If,
under these conditions, the pupil of the eye, instead of being placed directly
opposite the aperture in the eye-end, (in Fig. 5.) is moved more or less to the left,
it is evident that all the light coming through the aperture no longer enters the eye
and that only a fraction of it (which may be exceedingly small if the eye is moved
sufficiently to the left) enters the eye. The consequence is that both the spot and
the white ring are sufficiently diminished in brilliancy, both in an equal degree, to
enable the eye to loose sight of the difference of colour and to balance the bright-
ness of the spot and ring with facility.

page_text: When the photometer readings are in the higher numbers, the whole aperture is
used by the eye.
Each brand of sensitive plate must be subjected to trial exposures at the same
time that photometer readings are obtained, and, after the right exposure has been
obtained for any particular photometer reading, then the right exposure is known
for any other reading of the photometer.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that
what I claim is
1st. The use, for the purposes herein described, of the Standard Parliamentary
candle in conjunction with the opal or porcelain disc O, as a sufficiently constant
source of light.
2nd. The use of the slide S-S as a means of varying the light falling on the
diaphragm D in a known degree.
3rd. The use of the white, perforated, diagonal diaphragm.D in conjunction with
the lens (1), as a means of rendering light from any part of the photographers'
subject, measurable in terms of the light received from the illuminated disc O.
4th. The arrangement of the lamp L-L-C substantially as, and for the
purposes, herein described.