66 
Marks 
*SpectPO-Polariser after Rollett (Zeitschrift fiir Instru- 
mentenkunde, Jahrg. I, p. 366) as modified by Dippel, for deter- 
mining the character of double refraction in microscopical speci- 
mens for particular wave lengths. Combination of two flint-glass 
prisms PP, arranged to produce a deviation of ^0^, having on 
one side a movable slit Sp and a collimator lens C, on the other 
(at 0) a microscope objective which, from below, projects a real 
spectrum on the specimen under observation. Scale tube Sk 
on the prism casing, with mirror, collimator lens and a scale 
divided and numbered according to the wave lengths ; by reflexion 
from one surface of the prism a real image of this scale is pro- 
jected together with the spectrum into the plane of the object. 
The edges of the slit are moved symmetrically by a double 
threaded screw so that the centre-line of the slit remains station- 
ary. A Prazmowski prism Po mounted on a movable arm k in 
front of the slit serves as a polariser, and between it and the 
slit is a revolving ring G serving to receive selenite films for 
producing interference lines in the spectrum. An A, B, C or D 
objective is used to project a spectrum of the desired dimension 
and is screwed to the prism case by the narrow gauge thread on 
the lens mount. (Fig. 25.) 
This apparatus is arranged to connect with the frame of the 
Abbe illuminating apparatus by means of the centering adjustment 
supplied with the former. The adjustment in a vertical direction 
is effected by means of the rack and pinion, while the transverse 
adjustment of the spectrum is obtained by the centering collar. 
Including two selenite films for red of the second and third 
order 
340. 
