Improvements in Double Prisms for Totally Reflecting Refracto- meters.
Year: 1892

page_text: Date of Application, 10th Dec., 1892-Accepted, 5th Aug., 1893
COMPLETE SPECIFICATION.
Improvements in Double Prisms for Totally Reflecting Refracto-
meters.
We Professor ERNST ABBE, Doctor of Philosophy, and CARL ZEISS, Optician,
both of Carl-Zeiss-strasse at Jena in the Grand-duchy of Saxe-Weimar, German
Empire, 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:-
This invention relates to double prisms for employment in the construction of
totally reflecting refractometers and has the effect of enabling the prism itself to
produce the achromatisation of the critical line of the total reflection.
It is known that the differences which exist between the refractive power of
liquids, or semi liquids, can be accurately ascertained and measured by enclosing
the substance to be observed in the form of a thin layer or film between two glass
prisms the angles of which are equal but opposite, and determining the angle of
incidence under which the light has to be caused to pass through these two prisms
to undergo total reflection upon the intermediate layer of liquid.
This method has already been applied to practical purposes such, for example, as
the determination of the degree of purity of oils and fats, the degree of concen-
tration of solutions or the like.
Let it be supposed that the liquid constituting the aforesaid intermediate layer
possesses a refraction inferior to that of the glass. If, under these conditions an
illuminated surface be looked at by the medium of a double prism containing the
said liquid in the form of a thin layer between its two constituent prisms, one half
of the field of view appears light, whilst the other half is perfectly dark owing to
the total reflection of the luminous rays. By the aid of a telescope it is possible
to observe, either upon a scale arranged within the field of the eye piece or upon a
graduated are of the circle on the centre of which the telescope turns, the direction
in which the critical line of the total reflection is placed relatively to the faces of
the prism and finally to deduce therefrom the refractive index of the liquid
constituting the intermediate layer.
The apparatus which have heretofore been based upon this method of observation
in view of practical purposes and which are termed "totally reflecting refracto-
meters," present the defect that the critical line of total reflection is not achromatic.
This is due to the fact that the position of the said line is different for the
differently coloured constituents of white light. In order to obtain an accurate
observation of this critical line it is necessary to have recourse to homogenous or
mono-chromatic light or, in the case of white light, to complicate the apparatus by
providing it with a special arrangement constituting a compensator acting so as to
compensate for the dispersion of colours occuring when the reflection is total by a
prismatic dispersion.
The improvements forming the object of the present invention enable these
difficulties to be overcome in refractometers employed for verifying or examining a
definite substance or a definite class of substances such for example as a definite
kind or class of oil or fat, a definite kind of solution or the like. In this case the
achromatisation of the critical line of total reflection can be obtained by the double
prism itself and this with a degree of precision sufficient for all practical purposes.
For this purpose it is simply necessary to construct the double prism in such a
manner that when the reflection is total the dispersion taking place at that moment
on the limit between the glass and the liquid will be neutralized by an equal but
opposed dispersion on the face of emergence of the prism on the side nearest to
the shearver

page_text: devices a critical line of the total reflection which is colourless so long as the
substance composing the intermediate layer remains unchanged.
The aforesaid condition for the construction of the prism can be realised with
any kind of glass usually employed in apparatus of this description but in order to
reduce as much as possible the angle at which the light is to be looked at through
the prism the latter should be composed of a glass of which the quotient 72
AN
Δη
approaches as nearly as possible the quotient N of the liquid or body to be
tested.
It is most advantageous to employ in the composition of the prism a kind of glass
with which the value of the aforesaid quotient approaches as nearly as possible
that of the substance in question.
In this case the necessary compensation can be obtained with a small angle of
incidence (i) or even with perpendicular incidence.
The following are given as examples of the construction of the double prisms for
refractometers according to our invention.

page_text: The double prisms constructed as hereinbefore described may be made of any
required dimensions. They may be provided with any kind of supports or cases
according to the purposes for which they are required such as cases made with
hollow sides or walls for the purpose of regulating the temperature by means of
hot water for example. A telescope is combined with the prism in the usual
manner. In order to measure differences in the refractive power of the substances
under examination we may employ a graduated scale combined with the lens of the
eye piece in the usual manner or we may utilize in like manner the rotation of the
double prism relatively to the axis of the telescope in combination with a graduated
arc of a circle enabling the angle of the said rotation to be read off. When the
substances to be examined possess the same refractive power but differentiate with
regard to their dispersive power it is sufficient to observe the coloured critical line
in place of the achromatised line and in this case the assistance of means for
reading off the results and adjusting the apparatus and the telescope may all be
dispensed with.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is
:
A double prism for totally reflecting refractometers in which the critical line of
the total reflection is for definite substances achromatised by the double prism which
is so constructed that the emergent rays of light produce a dispersion of colours
which is contrary to that occuring on the limit separating the glass from the body
tested when the reflection becomes total substantially as described.
Dated this 10th December 1892.
J. H. JOHNSON & Co.,
Agents.