A CENTIGRADE PHOTOMETER
Year: 1880

page_text: PROVISIONAL SPECIFICATION left by the said Domenico Coglievina at
the Office of the Commissioners of Patents on the 26th August 1880.
DOMENICO COGLIEVINA, Civil Engineer, of Vienna, Louisengasse, No. 19.
"A CENTIGRADE PHOTOMETER.”
My Invention has for its object a more accurate and reliable measurement of
light than can be obtained by the ordinary photometers, which compare the light
from any source with the light of a standard candle, or similar unit, by means of a
movable screen.
In my photometer I adopt as unit the length of the extreme
beam of a light source, and compare the intensity of two light sources by letting
the light from both sources fall from opposite sides upon a ball, which produces an
image upon a mirror placed vertically under [the ball (or in any other suitable
position). If the ball is lighted equally from both sides its image is simply a circle,
but if lighted unequally, the limit between the two opposite hemispheres is visible,
and appears on the mirror as a straight line.
In carrying out my Invention I place two horizontally movable cases or chambers
opposite each other in a camera obscura so that their distance from each other can
be increased or reduced. The source of light to be measured is placed in one of the
said chambers (chamber No. 1), in which it can be raised or lowered by means of
a vertically movable table, and in the back wall of the chamber is an orifice and a
lens admitting light to a prism or reflector, which throw the light upon another
reflector. By means of repeated reflection the length of the beam of light from the
source to the object upon which it is finally thrown (preferably a ball placed in
the camera obscura between the two chambers) can be made very considerable, for
instance, 8 meter. When the light has reached the said object the source of light in
chamber No. 1 is turned down or otherwise diminished, until the beam is just strong
enough to touch the said object, this strength of light being adopted as a unit for
the subsequent operation.
In the front of chamber I. (towards the inside of the camera obscura) is
another opening, which, by means of a lens and suitable reflectors or prisms, admits

page_text: a pencil of light upon a horizontally movable ball, having a mirror below it. From
chamber No. 2, which contains another source of light and has a similar front
opening, a pencil of light is admitted to the opposite side of the said ball. The
length of the beam from the first light source to the ball is less than that from the
opposite source, and may be taken as half a meter. If the light thrown upon
the ball from the second source is stronger than that from the first source, the
second chamber is moved back, and with it a pointer movable along a scale, until
the intensity of light is the same on both sides of the ball, and the scale thus
indicates the intensity of the second light. If the second light is still too strong
after the pointer has arrived at 10, the second light source is diminished until the
ball is lighted equally on both sides, that is to say, until the second source is just
ten times as strong as the first; and then the second source is taken as measure of
light for the first source, by reversing the relative positions of the ball to the two
sources of light, that is to say, by placing the ball nearer to the second chamber,
the first source of light being fully turned up (or increased to its full strength.)
By repeating the operations until the first light source cannot be further
increased, and observing the graduated scale, the strength of the first source of light
can be measured.
The two chambers may be moved on rollers or by a rack and pinion, or in any
other suitable way; the height of the tables may also be increased in any suitable
way; for instance, by rack and pinion; and, instead of a ball as limit of the standard
length of beam, any applicable sensitive substance may be used.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Domenico Coglievina in the Great Seal Patent Office on the 25th
February 1881.
DOMENICO COGLIEVINA, Civil Engineer, of Vienna, Louisengasse, No. 19.
"A CENTIGRADE PHOTOMETER."
The inaccuracy of measurements obtained with ordinary photometers results,
partly from the adoption of a conventional unit, standard candle or standard lamp,
which does not produce a constant amount of light, and partly from the fact that
the effects of two light sources upon two different surfaces as produced in the usual
application of a screen cannot be compared with the desired accuracy.
In order to avoid these principal defects, the optical instrument in question
is based on the principle that a light source may be taken as a standard of
measurement, if its extreme beam attains a given invariable length (a), and also on
the principle, that a sphere (K) is illuminated equally by two light sources
(L, and L.) placed on opposite sides of it, if the image of this sphere on a
mirror (S) placed vertically under the latter does not show a border line or limit
between the sides illuminated by the two different sources, but appears as a uniformly

page_text: 8" "pp
and required to be measured, a beam of the light is conducted, first by the convex
lens, and then by the rectangular glass prisms II. to III. placed inside the photo-
meter box or camera obscura, so far that its length, until its contact with the
sphere F enclosed in the chamber F₁, amounts to a = 8 meter. By means of a
regulating cock, which may be placed directly under the light source or upon the
table of the photometer (in which case an india rubber pipe connects it with the
light source), the light L, is diminished until the said beain L₁, F, only touches the
sphere F. The flame thus regulated serves as a standard flame, and the beam L₁, F,
of the constant length a as a standard beam.
This standard flame is now transferred the the left side of the apparatus so as to
send through the convex lens l, and the prisms I. and IX., a beam of the likewise
constant length b 500 millimeter upon the opaque sphere K in the position of K₁.
The degree of illumination thus produced on the right hemisphere of K₁, I call one
degree of intensity,
=
In order to ascertain the full intensity of the light source L, in question I proceed
as follows:
If a second light source L. (which may have any strength, but must be capable
of regulation) is placed in A₂, this source illuminates the left side of K, through the
lens, and through the prisms X and XI.
Supposing now the chamber A, to be shifted towards the right side until the
distances of the two chambers A, and A, from K, are equal; supposing further, that
in such a position of the two light sources the sphere K, is illuminated on both sides
in such a manner that an equally illuminated image is produced on the plane mirror Ss,
it follows, that the intensity of L, is equal to one degree. However, if this is not
the case, and if the left hemisphere of K, is brighter than the right side, it is
necessary, for obtaining equal illumination of K, on both sides, to remove the
chamber A together with its light source towards the left, until in S, the limit
between the two differently illuminated sides disappears. To avoid the necessity
of giving to the apparatus an excessive length, its dimensions are such that A, can
only be shifted to the left as far as is required to cause a pointer to show 10° on a
scale applied to the outside of the photometer, said pointer moving simulta-
neously with the chamber, the total travel of which is 1081 millimeter. If even
in this position of the chamber A. the left hemisphere of K₁ is brighter than the
right one, it follows that the illuminating power of L, is stronger than 10°. If this is
the case, Le is first reduced to 10°, that is to say, it is turned down until the limit
or border line in the image S on the mirror disappears, subsequently K, is placed
in the position of K, L₁ turned up to its full strength and then compared with the
light source Le, the intensity of which (10°) serves now as unit for the further
measurements, in the same manner as has just been explained with L, and L₁
respectively. By this operation L, can be measured up to 100°. If now this
intensity, which is shown on the right arm of the scale by the corresponding pointer,
is taken as unit, L, may be increased to 1000° in a manner analogous to that
described, and the measurement of L, may be continued until the intensity of the
light source in question admits no further increase, that is to say, until pointer has
arrived at a point on the scale at which the above mentioned border line or limit
on the mirror disappears.
From this description it will be seen that the instrument in question renders
any conventional unit of light quite unnecessary, because it produces in every case
this unit by means of any other light source L., since it converts the light source L₁
in question into a standard flame by maintaining the constant length of the standard
beam of light.
The instrument is fitted with the necessary accessories according to the nature of
the light to be measured (gas light, daylight, &c.) If constructed for measuring
gas light, I call it centigrade-photometer, and heliometer if it is used for measuring
day light, the latter being chiefly serviceable to photographers. In the heliometer

page_text: the regulating cock is replaced by an opening applied to the front of the
chamber, and capable of being regulated by means of a graduated slide.
The motion of the chambers A₁, A., can be effected by means of rollers, slides, or
racks and pinions.
The Drawings referred to in the foregoing description represent an elevation
(Figure 1), a longitudinal section (Figure 2), and a horizontal section (Figure 3) of
a photometer. The light sources L₁ and I are supported by tables p,p, which can
be raised or lowered by means of racks 2,2, and pinions r₁, T. Instead of this
lifting mechanism, a spindle may be used which carries at the top the regulating
cock. I., XI., are prisms, the number of which may be varied according to the size
and shape of the photometer.
The ball F for ascertaining the limit of the standard beam is preferably made of
phosphor.
The same purpose may, however, be attained by a solid ball of other materials, or
by a substance upon which the contact with beams of light produces a distinctly
visible effect, for instance, a solution of quinine, tincture of curcuma or turnsol,
uranglass, fluorspar, &c.
With the two separate arms of the scale a centigrade scale is obtained, which,
by reason of the peculiar construction of the instrument, need only be multiplied
with 10 to be continued at will, without losing the original accuracy even in the
highest degrees, because the distance between the mark corresponding to 9° and
that corresponding to 10°, therefore also that between 99° and 100, &c., is always
81 millimatore