ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
741 
Tlie arrangement of Pulfrich is shown in fig. 88, that of the author 
in fig. 89. 
Against the arrangement in fig. 88, the author raises the objection 
that the rays coming from the liquid G x have a much longer path to 
travel through the prism P than those from the liquid G 2 which only 
pass through the reflecting prism p. This difference of path amounts 
to 4 cm. if the distance between the centres of the liquid columns G x and 
G 2 is 5 cm. 
The advantage, on the other hand, of the author’s arrangement, which 
at first sight appears so unsymmetrical, is that it is really optically 
perfectly symmetrical, since the lengths of path of the rays in the prisms 
P x and P 2 are the same, and the number of reflections is the same. 
Photometric experiments made by the author on Jena flint glass 
(No. 86) showed that the coefficient of absorption for 1 cm. thickness 
was 0*982, and for 4 cm. 0*930. Thus if this glass were used in the 
Pulfrich colorimeter with the centres of the two glass vessels 5 cm. 
apart, there would be an error of about 7 per cent, owing to the strong 
absorption in the prism P. 
The absorption also varies throughout the spectrum ; there is a 
considerable increase in the loss of light from the red to the violet end. 
In the Pulfrich arrangement, therefore, for different coloured solutions 
a different factor of correction would be necessary. The author suggests 
that the equality of path of the rays coming from the two liquids might 
be easily effected in the Pulfrich arrangement by increasing the length 
of the prism p downwards. 
(4) Photomicrography. 
Photomicrograms of Ice and Snow Crystals.* — Herr Neuhauss 
describes the method adopted by Redner for taking photomicrograms of 
ice and snow crystals. The apparatus was set up out of doors. The 
source of light was a small petroleum lamp and the objective a pro- 
jection-system of 31 mm. focal length (Hartnack). The linear magnifi- 
cation varied between twelve and twenty times. A concentrated alum 
solution, kept from freezing by the addition of rock-salt, served to 
absorb the heat-rays. Altogether twenty pictures were taken at —5 to 
— 10° R., five of ice and the other fifteen of snow crystals. Single pictures 
showed up to ten different forms of crystals. 
The Optics of Photography. j — This second part of Dr. Vogel’s 
handbook runs to 367 pages and has numerous figures with a coloured 
frontispiece illustrating the Vogel-Kurtz process of printing in three 
superposed colours. The subjects of photographic optics, viz. the methods 
of forming images, faults of lenses, intensity of illumination, &c., is dealt 
with in seven chapters forming an appendix to the work. The main 
portion of the book, consisting of 31 chapters extending over 266 pages, 
treats of such subjects as Intensity of light and Lambert’s law, 
photometry, standards of light, Methe and Michalke’s law of photographic 
reciprocity in developed films, sources of artificial light for photography, 
* SB. Gesell. Naturforsch. Freunde, 1893, pp. 18-9. 
f ‘Handbucli der Photographie, II. Theil : Das Licht im Dienste der Photo- 
graphie und die neuesten Fortschritte der Photographischen Optik,’ by Prof. Dr. H. 
W. Vogel, Berlin, 1894. See Nature, 1. (1894) pp. 589-91. 
1894 3 E 
