ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 649 



then less dazzling than those of mercurial photochromes. The author 

 has systematically studied fringes due to deposits of silver by stationary 

 waves. He shows that if the gelatin surface were absolutely parallel to 

 the glass, the appearance, seen from the glass side, would be only a 

 uniform tint due to the aggregation of strata of wave-length A. But, 

 practically, perfect parallelism is never obtained, and hence numerous 

 gelatin surface planes must cut the glass plane. This fact, he shows, is 

 sufficient to account for the effect produced. 



Photography of Very Translucent Diatoms at High Magnifica- 

 tions.* — The President of the Quekett Club, after describing the 

 difficulties encountered in the above research, recommends the following 

 method. A first negative being taken on a rapid plate, say at some 

 thousand or more diameters, is developed, preferably with hydrokiuone, 

 to obtain as much contrast as possible. If it is a good one, showing the 

 dots or secondary markings sharply focused, it is left to dry. When 

 examined it will be seen to show the veil which causes so much difficulty ; 

 perhaps such will be well seen around the dots, and will give them the 

 appearance of being immersed in a bath of fog. Perhaps the print may 

 show this defect more definitely than the negative itself. A fast plate is 

 then placed in contact (such a one as the " Flashlight " of the Imperial 

 Company), and the printing frame is waved once before a 16 c.p. 

 electric lamp, or some other powerful illuminant, placed about 2 ft. 

 away. This is developed as if it were a negative, i.e. by time. The 

 result is a very well exposed and developed positive, and not a very 

 thin and transparent one. The dots appear very plainly and sharply 

 focused, but there is a decided fog over the whole picture. This is 

 specially noticeable between the dots, and serves to muddle them up 

 in a very disappointing way. When dry, a copy of this is made upon a 

 slow plate, such as a process or a lantern-plate, and again developed by 

 time. This becomes the second negative. Even a cursory glance shows 

 at once how much brighter it is than the first taken direct from the 

 object ; but when the print or lantern-slide is taken from this the im- 

 provement becomes very apparent. 



(5) Microscopical Optics and Manipulation. 



Whittakeb, e. T.— The Theory of Optical Instruments. 



Cambridge : University Press, 1907, viii., 72 pp. 



(6) Miscellaneous. 



Influence of the Medium on Brownian Movements.! — V. Henri 

 studied these movements by means of photomicrographs obtained kine- 

 matographically with magnifications of 600 diameters. The medium 

 used was diluted latex, to which were added increasing quantities of 

 hydrochloric or acetic acid, of soda, urea, and alcohol. The results ob- 

 tained were that the Brownian movements are slowed by the addition 

 of a coagulating agent before the phenomena of coagulation are apparent, 



* Journ. Quekett Micr. Club, 1908, pp. 243-6. 

 t Comptes Renclus, May 18 and July G, 1908. 



