ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



707 



The holder consists of an upper band r x (fig. 110), set round the 

 objective mount, and a lower band c 2 encircling the back half only of 

 the front lens mount ; both these bands are held together by the 

 springs d lt d 2 . The lower band can be moved upwards by means of 

 the screws e l5 e 2 . The cuvette is placed in two sections on the under- 

 side of the band c 2 , and is then held fast by two springs secured by 

 screws. The cuvette is again shown in fig. Ill, and its two quartz 

 windows must be so arranged, that one faces the light source and the 

 other is parallel to the front lens of the objective. The screws e are 



Fig. ill. 



turned so as to bring cuvette and baud to a position such that the 

 quartz window is about 0*2 mm. from the front lens. The water- 

 immersion is filled by injection, and the superfluous water carefully 

 removed from the quartz window facing the light source. Quartz is 

 recommended on account of its resistance to fluids, and as being in its 

 molten state free from double refraction, and giving a sharp image of 

 the slit. The ends of the cuvette are connected by indiarubber coup- 

 lings with the thistle funnel g and the delivery tube h (fig. 101)), the 



20 — 



JUL 



Fig. 112. 



delivery tube being closed by a pinchcock k, and led away by a second 

 coupling i to a final delivery tube. The pinchcock is only opened 

 when it is desired to empty out the liquid in the funnel and cuvette. 

 The attachment of the funnel by its holder / is made after the insertion 

 of the objective. 



The illuminated fluid is now gauged, so that its breadth is read off 

 with an ocular micrometer as shown in fig. 112. The precision slit- 

 head is next rotated 90° into its position, and the proper slit-breadth, 

 corresponding to the depth of the observation, is then projected in the 



