ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 363 



Zsiginondy for ultra-microscopic observations. Its constituent parts were 

 (1) a carbon arc light, (2) a condenser, (3) a vessel filled with alum 

 solution for the absorption of the heat rays, (4) a diaphragm, (5) a 

 Microscope, and (6) a specially constructed camera. The first four were 

 taken from the large microphotographic apparatus of Zeiss. The Micro- 

 scope was also by Zeiss. The special camera was a four-sided metal 

 receptacle, 2 cm. by • 8 cm. by 1 • 8 cm., in which there were five round 

 openings ■ 3 cm. in diameter. The opening in each; of the smaller 

 surfaces was prolonged into a metal tube. The openings in the narrow 

 surfaces were placed opposite each other. The fifth opening occupied the 

 middle of the broad upper surface. The middle point of the three last 

 lay in one plane. The light passing through the condenser, the alum- 

 containing vessel and the diaphragm, entered the anterior opening of the 

 camera, which latter was placed on the Microscope object-wise. The 

 illuminated contents of the camera can then be studied through the 

 upper opening. By means of this apparatus various objects were 

 demonstrated, and among them the Bacillus dysenteries (Shiga) in 

 normal saline solution, with a magnification of only 31 diameters. 



Filtration of Ultra- Violet Rays through a Selection of Jena 

 Optical Glasses.* — H. A. Kriiss has investigated this subject in con- 

 nection with samples of glass supplied him by Schott and Co. These 

 samples represented the kinds of glass most frequently used in optical 

 instruments, and comprised the catalogue numbers 3004, 2000, 2090, 

 304G, 1800, 2572, 3111, 3013, 2563, 2625. Each sample was, more- 

 over, supplied in the three thicknesses. The results are tabulated in 

 the following manner : for glass of 1 mm., A = 309 to 384 ^ • for 

 glass of 10 mm., X = 309 to 434 fi/i ■ for glass of 100 mm., A = 309 to 

 480 /xfx. A full account of the method, apparatus and theory is given, 

 as well as an introductory bibliography. 



Optical Properties of Vitreous Silica.t — J. W. Gifford and W. A. 

 Shenstone point out that the properties of vitreous silica suggest that 

 it is not unlikely to play an important part in optical work. Its com- 

 position is definite, that is to say, it is not liable to those minute 

 variations which make it impossible to produce with certainty two 

 meltings of glass, which exhibit no sensible difference in their optical 

 properties when tested by a first-rate spectrometer. Hardly any cor- 

 rosive fumes, except those of fluorine -and hydrogen fluoride, attack 

 silica, and it is indifferent to most ordinary solvents. It is as trans- 

 parent to ultra-violet radiations as quartz, but is not doubly refracting 

 like that substance. Although it is a little difficult to prepare vitreous 

 silica in large masses, this difficulty can be surmounted, and the supply 

 of the substance is not limited like that of fluorite. In short, vitreous 

 silica places at our disposal a really standard glass. Its refractive index 

 is low, and its dispersive power is sensibly greater than that of quartz. 



The authors describe the method of manufacture, which involved a 



* Zeit. f. Instrumentenkunde, xxiii., July 1903, pp. 197 207 ; August 1903 

 pp. 229-39 (7 figs.). 



t Proc. Koy. Soc, Ixxiii., No. 491, pp. 201-8 (3 figs.). 



