ZOOLOGY AND BOTANY? MICROSCOPY, ETC 



401 



pushing in the sliding tube and closing the clamp H. It is convenient 

 to graduate the sliding tube, by direct comparison with the ground- 

 glass, in numbers representing the corresponding camera-lengths. When 

 this graduation has once been made, it is sufficient, in taking a photo- 

 graph, to fix the telescope for the proper camera-length and focus, the 

 microscope so that the image is clearly defined on the cross-wires. As 

 soon as the focus is ascertained to lie steady and satisfactory, the dark 

 slide having been opened before focussing, the exposure can be made by 

 simply turning back the milled head C, so tliat the mirror moves into 

 the vertical position. 



The lenses described are of the simplest type, and have proved 

 quite satisfactory in working ; the field is, however, rather narrow, about 

 one-third of the diameter photographed being visible at once. Mr. Swift, 

 by whose firm the apparatus was made, has suggested that the eye-piece 



of the telescope might be provided with a field-lens, and no doubt the 

 full field could thus be obtained ; at the same time it would certainly be 

 well to make the objective achromatic. No difficulty has been found to 

 arise from the use of the mirror, which is a common plane " galvano- 

 meter mirror," though the best result would no doubt be given by a 

 silvered right-angled prism. For ordinary purposes the simple form 

 described has proved sufficient and represents a considerable saving of 

 expense in comparison with the more correct construction. J. E. B. 



(5) JMicroscopical Optics and Manipulation. 



Optical Properties and Theory of Colour of Pigments and Paints. 

 — H. E. Mekwin {Froc. Amer. Soc. for Testing Materials, Philadelphia, 

 1917, XVII., 2, 1, 2). The hue, purity and brightness of light diffused by 

 a pigment or paint depends upon the refractive index, colour absorption, 

 size, shape and texture of the pigment grains, and upon the refi'active 

 index, colour, and continuity of the vehicle ; and also upon the distri- 

 bution of the grains in the vehicle. 



A black pigment to be most effective optically should have (1) 

 a refractive index equal to that of the surrounding medium, and (2) 



2 E 



