THE PHOTOGRAPHIC TECHNIQUE I3 



familiar with the Golgi preparations know that the tissue surrounding the silver impregnated 

 cells and processes frequently has a decidedly yellow colour, which more or less rapidly darkens, 

 and as it was desired to reproduce the effect of the cells and their processes upon a nearly 

 or quite colourless background, it was found advisable to use both colour-sensitive plates and 

 an orange-yellow screen. In previous work it had been the custom to use as a colour screen 

 a glass cell containing a solution of one of the Tropasolins. The use of such a cell is not, 

 however, unattended with difficulties ; it is somewhat troublesome to make up anew each time, 

 as so doing involves the use of the spectroscope if accuracy be desired ; it is subject to change 

 from dust, from evaporation, and in a short time from precipitation of the dye from its aqueous 

 solution. In lieu of the glass cell, therefore, an unexposed lantern slide plate was fixed in 

 sodium thiosulphite and well washed ; when dry it was placed upon a level surface, gelatine- 

 coated side upwards ; upon this was poured a strong solution of Tropaeolin in alcohol, which 

 was allowed to evaporate protected from dust ; Canada balsam was then poured over the dyed 

 plate and a cover glass applied. The plate thus prepared was allowed to harden under pressure 

 in a vertical position. A darker coloured screen was made by putting two dyed plates face 

 to face in the same manner. The screens so made have proved very convenient, show scarcely 

 perceptible fading or change during two years of constant use, and do away entirely with all 

 the trouble incident to glass troughs and fluids. 



The optical apparatus consisted of a Zeiss' Abbe condenser specially made for photography 

 of i N. A. and achromatic, and of the following objectives : a Zeiss 2 mm., 3 mm., 4 mm., 

 8 mm., and 16 mm. apochromatic lenses, a 35 mm. projection lens of Zeiss, a ^-in. Bausch 

 and Lomb, a i-inch Ladd of London, and a 3-inch Ross of London. No eyepiece was 

 employed with the dry objectives, the camera length being 140 cm., which gave a magnifica- 

 tion of from 25 to 190 diameters; with the oil immersion apochromats 2 and 3 mm. a projection 

 ocular No. 4 was used and a camera length that gave from 1000 to 500 diameters by stage 

 micrometer measurement. It was found when the image was thrown upon the screen that no 

 matter how carefully the light was centred there would remain diffraction spectra, and the 

 phenomenon known as halation manifested itself in this peculiar way, the stain being an 

 impregnation of the chromate of silver and absolutely opaque, the light would be refracted 

 around the edges of the cell bodies and large processes, and would appear on what should 

 have been a perfectly black image as little tufts and jets of white (plates Nos. XIV. and 

 XXXIV. are examples of this being among the first taken). In order to obviate both these 

 difficulties a plate of finely ground glass was inserted, with its plane at right angles to the 

 optical axis of the system, between the condenser and source of light, and placed so as to be 

 within the focus of the condenser when both condenser and objective were focussed upon the 



