ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



Ill 



(fig. 12) carries two pieces of cobalt glass of different tints in a tele- 

 scopic horizontal arm, sliding upon a vertical pillar attached to a heavy 

 base ; the other (fig. 13), has a hinged arm and bull's-eye. 



Fig. 12. 



Fig. 13. 



Swift's Double-Image Prism for Petrological Microscopes.*- — 

 This accessory is shown in fig. 14, and will be found extremely useful 

 for viewing small dichroic crystals. The two images are seen side by 

 side in the field, and one rotates round the other when tbe prism is 

 turned round the eye lens of the ocular. The images differ according 



Fig. 14. 



to the nature of the crystal mineral and the direction in which the light 

 passes through it. A thin plate of brass, with a number of small aper- 

 tures, is inserted in the eye-piece for the purpose of reducing the field 

 to a size smaller than the crystal under observation. 



I (4)i Photomicrography. 



Rose, L. K.— Photomicrography of Metal?. 

 [An historical and practical paper.] 



Photographic Journ., xliii. (July 1903) pp. 195-9. 



* J. Swift & Son's Catalogue, London, 1901, p. 40. 



