ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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each other, one of which, I, must he plane polished, while the face II 
need be only so far polished that light can enter. Between the object 
and prism face is a thin film of a liquid with a higher refractive index 
than the object. 
The improvements which Dr. Pulfrich has introduced into the new 
instrument render it serviceable for almost all refractometric and 
spectrometric investigations, viz. : — 
(1) For the determination of the refraction (w D ) and the dispersion 
(difference of the indices for the Fraunhofer lines C, D, F, and G') of 
transparent, liquid, and solid (single and doubly refracting) bodies. 
(2) For the investigation of liquids at high temperatures , e.g. of 
bodies which only become liquid at high temperatures. 
(3) For the determination of the differences of refraction and dis- 
