A New Instrument for the Measurement of the Refractive Index of Light in Liquids.
Year: 1892

page_text: Date of Application, 17th Mar., 1892
Complete Specification Left, 15th Dec., 1892-Accepted, 17th Mar., 1893
PROVISIONAL SPECIFICATION.
A New Instrument for the Measurement of the Refractive Index
of Light in Liquids.
CHARLES FÉRY of 69, Great Ormond Street, London W.C. Teacher of
Physical Science, do hereby declare the nature of this invention to be as
follows:-
The essential & novel part of this invention is a trough the sides of which A, B,
are formed of two strips of glass cut out of a spherical lens; a cross-section of
these is shown in the sketch. The plane surfaces of these strips form the two
opposite inner sides of the trough & are placed at an angle to one another; the two
ends C, D, & the bottom of the trough are of glass or other material. A liquid
poured into the trough assumes the form of a prism of constant angle two
opposite sides of which are bounded by the solid prisms of glass of varying angle,
as shown in the figure.
To use the trough it is placed between a source of light from which a narrow
vertical band of rays of light proceeds horizontally rendered parallel by appropriate
means & a telescope by means of which these rays can be received by the eye & so
that the direction of the band of light is at right angles to the plane bisecting the
prism lengthwise.
The trough can be moved backwards & forwards horizontally in the direction of
the imaginary bisecting plane above mentioned by means of a screw or otherwise &
in such a way that the amount of movement can be measured.
When the trough is filled with a liquid the refraction of the light caused by this
liquid is, when the trough is in a certain definite position, annulled by the refraction
caused by the prisms of glass and the light passes into the telescope & is received
by the eye; a liquid of known refractive index is introduced into the trough & the
position of the trough is noted when the eye receives the light through the
telescope; another liquid of another known refractive index is put in the place of
the first and the position of the trough when the light now reaches the eye
through the telescope is noted. From these two definite points so determined a
scale of equal units is easily constructed & the unknown refractive index of a liquid
determined by introducing the liquid into the trough & reading on the scale the
position of the trough when the band of light passing through it & the telescope
reaches the eye.
Dated this 16th day of March 1892.
COMPLETE SPECIFICATION.
A New Instrument for the Measurement of the Refractive Index
of Light in Liquids.
CHARLES FÉRY 40 Rue Nansouty, Paris late of 59, Great Ormond Street,
London Teacher of Science do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:-
My invention consists of a hollow prism of glass (see the drawing filed with my
Provisional Specification), in which the liquid to be examined is placed, the two
sides of which prism are formed of similar slips of glass A & B, cut out of a plano-
convex lens, and placed as shown in the figure.
To use this prism a collimator giving a pencil of parallel rays of light is placed
ID: 037

page_text: in relation to an ordinary telescope furnished with cross-wires so that the image of
the slit in the collimator coincides with the cross-wires in the telescope; the prism
is placed on a stage to which the collimator and telescope are rigidly fixed between
these two in such a way that an imaginary vertical plane bisecting the prism along
its length is at right angles to the ray of light going through the collimator &
telescope. Means are provided that the prism can be moved forwards & back-
wards on the stage in the direction of its length and a pointer connected with the
prism moves along a scale on the stage as the prism is moved. The scale is marked
as follows:
Having placed a liquid of known refractive index (a) in the prism, the prism is
adjusted, until when looking through the telescope the slit in the collimator coincides
with the cross-wires in the telescope. The position of the pointer on the scale is
pow noted & that point on the scale is marked with the number denoting the
refractive index (a) of the liquid under examination. Having replaced the first
liquid by another liquid of another known refractive index (b) the prism is again
adjusted until the image of the slit in the collimator coincides with the cross-wires
in the telescope. The position of the pointer on the scale is again noted and this
point is marked with the number denoting the refractive index of this second
liquid (6). These two points being determined the scale is divided off in the usual
manner.
When now a liquid of unknown refractive index is placed in the prism & the
prism adjusted as detailed above the position of the pointer on the scale indicates
the refractive index of the liquid filling the prism.
To make clearer the description I have given above of the method in which my
prism of glass is used in conjunction with the necessary adjuncts, the collimator &
the telescope with crosswires, I append another figure, which I now proceed to
explain :
C is the collimator, in front of which is placed a source of light as described.
T is the telescope furnished with crosswires.
P is the hollow prism of glass.
St is the stage to which C & T are rigidly fixed.
M is the means provided, namely, a rack & pinion, by means of which P can be
moved backwards & forwards on St.