ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



99 



Fig. 23. 



Origin of the Davis Shutter.— It 

 will be found, on referring to the 

 Journal for 1882, p. 262, that an iris 

 diaphragm placed at the back of an 

 object-glass (now known as a Davis 

 shutter) was first suggested by Dr. 

 Royston Pigott in 1869, for reducing 

 the aperture of objectives. At that 

 date Dr. R. Pigott maintained that 

 wide-aperture objectives produced con- 

 fused images. 



Simple Form of Reflecting Pola- 

 riser.* — F. J. Cheshire mounts, in 

 the axis of the Microscope, a slip of 

 ground glass Gr (fig. 25), about lj 

 by 2£ in., at an inclination of 33^°, on 

 a short spindle A, capable of rotation 

 by a milled head B. The glass slip is 

 blackened with Aspinall's enamel on its. 

 back and ground side. This polariser 

 is mounted on the tail-piece of the 

 Microscope in the same way as the 

 usual mirror. Therefore when the 

 spindle A has been rotated so as to 

 bring the lamp-flame into view, the 

 light is reflected at the proper angle 



Fig. 24. 



* Journ. Quek. Micr. Club, Nov. 1902, pp. 353-4 (1 fig.). 



H 2 



