IMPROVEMENTS IN PHOTOMETRIC APPARATUS
Year: 1882

page_text: LETTERS PATENT to Stephen Henry Emmens, of Soho Square, in the County
of Middlesex, Actuary, and John Munro of West Croydon in the County
of Surrey for an Invention of "IMPROVEMENTS IN PHOTOMETRIC APPARATUS
"
PROVISIONAL SPECIFICATION left by the said Stephen Henry Emmens
& John Munro at the Office of the Commissioners of Patents on the
22nd December 1882.
STEPHEN HENRY EMMENS of Soho Square in the County of Middlesex Actuary,
and JOHN MUNRO, of West Croydon in the County of Surrey IMPROVEMENTS
IN PHOTOMETRIC APPARATUS
Our invention relates to photometrical apparatus, depending for its action upon
the employment of light absorbing media, and we construct our apparatus in
several modifications, corresponding to its varied applications.
Of such modifications, the first embraces a prismatic analyser of the light, which
is also a photometer. It consists of a prism (suitably of flint glass) through which
a ray of light from the source to be measured is reflected by a small sliding mirror.
The ray, when refracted by the prism, displays its spectrum upon a reflecting
screen. In front of the reflecting screen is a slip of glass graduated with fine lines
to measure the parts of the spectrum, and also to gauge its intensity by noting
the vanishing points of the lines. The whole is enclosed in a suitablo case.
The quality of the spectrum shows the constitution of the light, and its intensity
can be measured by the degree of its brightness in relation to the thickness
and absorbing power of the prism through which it has passed. The depth of
prism through which the beam is refracted may be varied by means of the sliding
mirror.
A Second modification is in a small and portable form, designed more especially
for use in regard to electric light. In this case we employ a prism or wedge of
neutrally tinted glass, which, by means of a suitable push or screw, is capable of
interposing a greater or less thickness of glass in the path of the beam of light.
The thickness of glass which absorbs or eclipses the beam, is a measure of the
intensity of such beam. A second transparent prism may be placed next tho
Paina 21

page_text: neutrally tinted one to keep the beam in a straight direction and obviate refraction.
The pair of prisms can be moved as one piece, or by means of suitable rack and
screw movement are capable of sliding across one another. Again, the neutral
prism may be withdrawn, allowing the transparent prism to cast a spectrum of
the beam; or a second neutral prism may be used in place of the transparent
prism, to give a double effect, and would be manipulated in the same way.
graduated scale is added to show the candle intensity of the light.
A
When the light is a powerful one, a very dark neutral glass may be used, or one
or more glasses of known absorbent power may be inserted in front of the
measuring prisms, to absorb a given intensity of the light, the remainder being
absorbed by the adjustable wedges. In the same way glasses of various powers
of absorption may be inserted to absorb certain rays, allowing others to pass and
be measured. A lens may be employed to give a small image or focus of the light
instead of a single beam or pencil of rays being measured. Any convenient object,
such as a white bead with black marks, or black bead with white marks, may be
mounted in front of the object piece of our instrument, and the general illumination
at any distance or different distances from the source of light, may be measured by
the reflection therefrom.
'
A third modification of our apparatus is where we attain the measurement by
means of a shadow cast on the wedges, of any convenient object, such as a wire
stretched across the objective. The obscuring of these shadows by the adjustment
of the wedges forms the means of measurement. Two shadows may be thrown
(as in Rumford's photometer) and a comparison of intensities of the lights which
produce them may be measured by the wedges, or by the adjustment of the instru-
ment, until a similar thickness of wedge absorbs both images.
It is obvious that our wedge device is susceptible of other modifications which
it is not necessary to specify, such as by the interposition of reflectors, absorbent
or otherwise, between the wedges and the beam.
In a fourth modification, we dispense with the wedges as variable absorbents
of the light, and we use liquid of known absorbent power in a telescopic tube and
reservoir. The telescopic tube has a transparent disc at each end, and its slide
is graduated, and the variable thickness of the absorbent liquid (taken from
or discharged into the reservoir which may be in the form of a second pipe)
determines the measurement in place of the varying thickness of a wedge or
wedges.
A fifth modification is where our absorbent media are arranged in the shape of
glasses on a frame (as in an ordinary actinometer or sensitometer) graduated
according to their absorbent power. By observing any beam at a given distance
from the source of illumination and noting the glass at which absorption takes
place the measurement may be taken.
A sixth modification is where we employ the absorbent properties of a single
mirror or of a series of mirrors fixed at proper angles, and noting the number of
reflections given of the beam.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Stephen Henry Emmens & John Munro in the Great Seal Patent
Office on the 22nd June 1883.
STEPHEN HENRY EMMENS, of Soho Square, in the County of Middlesex, Actuary,
and JOHN MUNRO, of West Croydon, in the County of Surrey,
"IMPROVEMENTS
IN PHOTOMETRIC APPARATUS
>>
Our invention relates to photometrical apparatus depending for its action upon
the employment of light absorbing media, and we construct our apparatus in several
modifications, corresponding to its varied applications.
In order that our invention may be the bettor understood, we now proceed to
describe the same with reference to the drawings hereunto annexed.
Our first modification, shown in sectional elevation in Figure 1, embraces a
prismatic analyser of the light, which is also a photometer.
The
It consists of a prism A. (auitably of flint glass) through which a ray of light from
the source to be measured, admitted through the small orifice B, is reflected by a
small sliding mirror C. The ray, (shown by a dotted line) when refracted by the
prism, displays its spectrum upon a reflecting screen D. In front of the reflecting
screen is a slip of glass E, graduated with fine lines, to measure the parts of tho
spectrum, and also to gauge its intensity by noting the vanishing points of the
lines. The whole is enclosed in a suitablo case, provided with an eye-hole F.
quality or range of refrangibility of the spectrum shows the constitution of the
light, and its intensity can be measured by the degree of its brightness in relation
to the thickness and absorbing power of the prism through which it has passed.
The depth or thickness of prism through which the beam is refracted may be varied
by means of the sliding mirror C, which is adjustable as to its position with regard
to the prism A, by the rack and pinion motion G, and as to its angle of reflection
by the screw H, moving the angle adjustment rod H¹ against the joint J on the end
of the carrying rack. The position and angle of the mirror, and therefore the
thickness of prism through which the ray is caused to pass, may be estimated by
the lengths of carrying rack G¹ and prolonged angle adjustment rod H¹.
A second modification is in a small and portable form, designed more especially
for use in regard to electric light. In this case we employ a prism or wedge of
neutrally tinted glass, which, by means of a suitable push or screw, is capable of
interposing a greater or less thickness of glass in the path of the beam of light.
The thickness of glass which absorbs or eclipses the beam, is a measure of intensity
of such beam. A second transparent prism may be placed next tho neutrally
Tho
tinted one to keep the beam in a straight direction and obviate refraction.
pair of prisms can be moved as one piece, or by means of suitable rack and screw
movement are capable of sliding across one another. Again, the neutral prism may
be withdrawn, allowing the transparent prism to cast a spectrum of the beam; or
a second neutral prism may be used in place of the transparent prism to give a
double effect, and would be manipulated in the same way. A graduated scale is
added to show the candle intensity of the light.
When the light is a powerful one, a very dark neutral glass may be used, or one
or more glasses of known absorbent power may be inserted in front of the measuring
prisms or wedges, to absorb a given intensity of the light, the remainder being
absorbed by the adjustable wedges. In the same way glasses of various powers of
absorption may be inserted to absorb certain rays, allowing others to pass and be
measured. A lens may be employed to give a small image or focus of the light
instead of a single beam or pencil of rays being measured. Any convenient object

page_text: such as a white bead with black marks or black bead with white marks may be
mounted in front of the object piece of our instrument, and the general illumination
at any distance or different distances may be measured by the reflection therefrom.
A third modification of our apparatus is where we attain the measurement by
means of a shadow cast on the wedges, of any convenient object, such as a wire
stretched across the objective. The obscuring of these shadows by the adjustment
of the wedges forms the means of measurement. Two shadows may be thrown,
and a comparison of intensities of the lights which produce them may be
mensured by the wedges, or by the adjustment of the instrument until a similar
.thickness of wedge absorbs both images.
In order that these modifications may be more clearly understood, we now
proceed to describe instances of the same, referenco being had to Figures 2. 3.
and 4. of the accompanying drawings.
Figuro 2 shows in front elevation, plan, and back elevation a form designed for
the pocket. A small casing B is fitted with an eye piece and lens L, the latter
being adjustable to get good definition. Past the eye piece slides a wodge W of
neutral tinted glass, mounted upon a light frame, which carries a graduated scale S.
This scale is graduated either in candle power or in arbitrary divisions which
'express the candle power by aid of a table. At the back of the casing B is a small
window or slit pane of ground glass or other partially transparent material g,
having a wire or fibre stretched across the pane. The light to be estimated enters
by this partially transparent pane and is seen through the wedge W by looking
into the eye-piece L, which is properly focussed. When the wedge is gradually
adjusted by means of the screw R, until a sufficient thickness is interposed to
almost completely extinguish the figure of the wire or absorb the light, (if no wire
be used); the reading of the scale attached gives the value of the light.
Though the foregoing is the form we prefer, yet other arrangements may be
adopted for instance, a fluid partially transparent pane may be used, as also a
wedge of tinted liquid; more than one wire may be employed; the light may be
received on white screens of opal glass or other material or reflected from mirrors;
a shadow of the wire may be used instead of the wire itself; tinted glasses of special
absorptive power may be interposed between the eye piece and wedge, or between
the aperture and wedge, to cut off a definite amount of light; but in every case
the wedge, absorbing light through a partially transparent pane, is employed.
Figure 3 shows in front elevation, plan, and back elevation, an instrument in
which two tinted wedges W are employed, actuated by the screw R. The
thickness of interposed tinted glass serves as beforo to determine the value of tho
light, and is indicated on the scale S. The variations specified in relation to
Figure 2 may occur in this form.
Figure 4 shows in front elevation, plan, and back elevation, our instrument 4
combined with partially transparent panes and mirrors or screens, for comparison
of two lights. B is the case; g g are two partially transparent panes, through
which the two lights are received; W is an absorbent wedge, adjustable by means
of the rack and pinion motion R; and M M are two mirrors or screens set at an
angle to receive the respective beams through the panes, and to allow of their 4
comparison through the eye piece L. The objectivo in front of the wedge is
turned to the stronger light, and the thickness of wedge, interposed by the rack
and pinion when the two reflections are equalized, as indicated on a graduated
scale, is the measure of the difference of the lights.
It is obvious that this form, is susceptible of other arrangements, such as the use 5
of two wedges, the use of a wire or shadow of a wire, or tinted fluid panes or
wedges, as specified in regard to Figures 2 and 3.
In a fourth modification, we dispense with the wedges as variable absorbents of
the light, and we use liquid of known light absorbent power in a telescopic tube
and reservoir. This wo show in Figure 5, in which T is the telescopic tube, closed 5
with a partially transparent or transparent pane g at one end, and by an eye piece L
at the other end. The slide of this telescopic tube is graduated, S, and the variable

page_text: thickness of the absorbent liquid between the observer and the objective (taken
from or discharged into the reservoir R) determines the measurement in place of
the varying thickness of a wedge or wedges.
A fifth modification is where our absorbent media are arranged in the shape of
glasses on a frame (as in an ordinary actinometer or sensitometer) graduated
according to their absorbent power. This we show in front and side sectional
elevation in Figure 6, where the instrument is flat in form, and in Figure 7 where
the instrument is circular. In a suitable frame or casing C and at one end thereof,
are mounted the superposed absorbing glasses g, each superposition being divided
off by partitions p. At the opposite end of the casing is the eye piece L. By
observing any beam at a given distance from the source of illumination, and noting
the glass or partition at which absorption takes place, the measurement may be
taken.
A sixth modification is where we employ the absorbent properties of a singlo
mirror, or of a series of mirrors fixed at proper angles, and noting the number of
consequent reflections given of the beam. This we illustrate in Figure 8, where a
single mirror is shown, and in Figure 9, where a series of two mirrors are
employed. In Figure 8 the instrument is shown comparing two lights, their
images being reflected from inclined mirrors M through a suitable tube T, on to a
mirror K, inclined at such an angle that the observer, through the eye piece L, is
able to observe the consequent reflections of each image. The inclined mirrors
may be dispensed with and the light received direct, where the value of one light
is to be calculated. In Figure 9 the image is received through t on to a mirror K,
which reflects it on to K, whence it is again returned to K, and so on until
absorption takes place. The number of consequent reflections in these cases is the
index to the value of the light.
In all our modifications we may employ wedges or panes of known light
absorbent power to moderate the intensity of the light to be measured.
Having now fully described and ascertained the nature and objects of our
invention, and the manner of its performance, be it known that what we claim as
novel and desire to protect by Letters Patent is :-
First. In a photometer and prismatic analyser, the combination of adjustable
mirror, refracting prism, reflecting screen, and optional graduated slip, suitably
encased, substantially as set forth.
Second. In a photometer the combination of an adjustable light absorbent wedge
or wedges, with scale, and partially transparent panes, and, optionally, with
illuminated screens or mirrors, the whole suitably encased and mounted,
substantially as set forth.
Third. In a photometer, the combination of telescopic tube and scale with light
absorbent liquid and suitable reservoir, substantially as set forth.
Fourth. In a photometer, the combination of superposed slips of light absorbent
material, with partially transparent pane and partitions and suitable casing,
substantially as set forth.
Fifth. The constrution of photometer consisting of inclined mirror or mirrors
giving consequent images of the light or lights to be measured, suitably encased,
substantially as set forth.
In witness whereof I, the said Stephen Henry Emmens, on behalf of myself
and the said John Munro, have hereunto set my hand and seal this twentieth
day of June in the year of our Lord One thousand eight hundred and