132 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



be used with the lens, which in this case should not be of too short focal 

 length (at least 30 cm.). 



The whole disposition of the instrument is seen from the figure (24), 

 in which d is the three-way piece, P the plate mirror, L the condensing 

 lens, Bl the diaphragms, C the cell, K the front part of the camera, 

 Ti and T3 tables on which rests the base-board carrying the camera and 



Fig. 25. 



optical bank, Tg the table on which stands the small table G carrying 

 the shutters A and B, and k the screw for regulating the tension of 

 the adjustment-cord, which, in this apparatus as in that of Prof. Strieker, 

 works not on the micrometer- screw of the Microscope, but on a second 

 micrometer-screw connected with the stage. 



For the adjustment the condensing lens is removed, and the path of 

 the beam of light reflected from the mirror is centered by means of the 



Fig. 26. 



two diaphragms of equal opening. In the figure is represented the 

 moment when the adjustment is just finished and the lens inserted, but 

 the diaphragm turned towards the mirror not yet replaced by one of 

 wider opening. The latter is chosen of such a size that it cuts off only 

 the zonal edge of the beam, and is situated at such a distance (at least 

 15 cm.) from the second diaphragm, that the light cone exactly passes 

 through the latter. With a condensing lens of 10 cm. opening and 

 focal length of 33 cm., diaphragm I. (with 70 mm. opening) is distant 

 62 mm., and diaphragm II. (with 28 mm. opening) 222 mm. from 

 the lens. 



As regards the time of exposure, for small Crustacea with a magnifi- 



