ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



123 



virtual image of the outer opening of the tuhe on the spot where is the 

 opening of the objective. 



The most important part of the lower piece is the mechanism of the 

 two slits, which are independent of each other and lie in the same hori- 

 zontal plane. The symmetrical movement of the edges is effecteol, as in 

 the author's micmspectral objective and in the spectral apparatus of 



Fig. 14. 



Bonders, by a single screw which carries two oppositely wound threads. 

 The mechanism of one of the two slits is shown in vertical section in 

 fig. 15. The edges p and p' are screwed on the blocks e and e', which 

 carry companion screws for the screw-threads on the common axis a. 

 The screw on e is right-handed, that on e' left-handed. To avoid back- 

 lash e and e' are kept apart by a spring not visible in the figure. The 

 axis a is firmly fixed on the strong metal frame m, so as not to be mov- 

 able in the direction of its length. Consequently, by rotation of a, the 



