ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 513 



small, and, moreover, it possesses the very great merit of getting rid of 

 the top light reflected down upon the surface of the object by the upper 

 surface of the cover-glass. 



(3) Illuminating- and other Apparatus. 



Magnesium Flash-light for Zoological Work.* — W. E. Watson- 

 Baker, in a paper read before the Photomicrographic Society, called 

 attention to a new source of light he had seen described by 8. B. Doten 

 in an American publication on photography called " Camera Craft." 

 The method was suggested by the fact that a small quantity of mag- 

 nesium-flash powder may give light equivalent to full sunlight upon 

 an object G ft. away from the flame. But the comparative slowness of 

 the magnesium flash stood in the way of perfect success. Further 

 attempts were therefore made by Doten, all depending on developing 

 a flame by the electric vaporization and ignition of a fine wire in free 

 air. The apparatus finally adopted was as follows. A little block of 

 J. M. Transite asbestos- wood carries two brass binding-posts f in. in 

 diam. and 1 in. high, with rounded tops. A strand of No. 31 B.and S. 

 gauge silver or copper wire is clamped beneath them, like a fuse in a 

 branch-block. A knife-switch, whose blade may be thrown into contact 

 at will very suddenly by a pneumatic release and a steel coil spring, 

 shoots a heavy current through the fuse, which is instantly vaporized ; the 

 vapour springs up from a slot in a block of transite placed over the 

 posts. The vapour shoots beyond the tops of the posts, while from 

 the top of each a jet of vapour pours out which is far more luminous 

 than that from the wire itself. It was found that this flame is cold in 

 the sense that it will not burn the hand. It is clear, smokeless, clean, 

 and most intensely brilliant. Many sorts of fuse-wire — copper, silver, 

 lead, aluminium, magnesium, and carbon — were tried between posts of 

 brass, carbon, and magnesium. On the whole, the 1-in. length of 

 magnesium wire between brass posts f in. in diam. and 1 in. high would 

 seem to be the best of all. With direct current of 110 to 120 volts all 

 the metals tested between any of the posts give flames uniformly sure 

 and brilliant. 



(4) Photomicrography. 



Senior, E.— Some Interesting Experiments in Photomicrography. 



[Under the above title the author describes the apparatus, adjustments, 

 accessories, and general details necessary for successful photomicro- 

 graphy.] Journ. Photomicr. Soc, iv. (1915) pp. 52-8 (many figs.). 



(5) Microscopical Optics and Manipulation. 



Theory of Diffraction in Relation to the Theory of Optics. f — J- W. 

 Gordon, in a lecture before the Photomicrographic Society, suggested 

 the term " perioptrics " as suitable for the study of the diffracted image, 

 and considered that it ranked equally with dioptrics and catoptrics as 

 main divisions of the theory of optics. After indicating by an experi- 



* Journ. Photomicr. Soc, iv. (1915) pp. 40-2. 



t Journ. Photomicr. Soc, iv. (1915) pp. 33-40 (1 fig.). 



