ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
123 
two ball-joints pressed tightly against the corresponding sockets of the 
tube /. In order to prevent any rotation of the tube /, there is a screw i 
on the lower socket, which slides in the same groove of the ball-joint b , 
Fig. 19. 
Fig. 20. 
as the rod h. As regards the rest of the apparatus, the exact adjustment 
is effected by the rack and pinion at d, and the lens itself fits into a 
spring socket, and is fixed by means of the screw e. 
Micropolariscope for Projection.* — Mr. F. C. Van Dyck describes 
the form of apparatus which he has used for projection work. The 
figure (fig. 21) shows the general arrangement, but does not indicate 
proportions. 
The light from an arc-lamp, in which an electro-magnet keeps the 
crater steadily directed towards the condenser, after passing through the 
pair of lenses constituting the condenser and through the alum-cell, falls 
in parallel rays upon the polariser. The parallel rays reflected from 
the polariser are converged by the second condenser-lens so as to come 
to a focus upon, or nearly upon, the object. A substage condenser is 
also sometimes necessary. As objective the author has used with good 
results a 7/8 bought of Queen and Co., and of higher powers a 1/2 of 
Tolies and a 1/4 of Bausch and Lomb. The analyser is a Nicol prism 
about 3/4 in. across the face. 
As regards the alum-cell, the liquid was about 5/8 in. in thickness, 
and consisted of a mixture of equal volumes of cold saturated alum solu- 
tion in water and strong glycerin. The polariser is made of twelve plates 
of as colourless a glass as possible. To prevent contact and formation 
* Amer. Micr. Journ., xvi. (1895) pp. 15-1-6. 
