14 ATLAS OF NERVE CELLS 







object; this gave a softly, evenly illuminated field, particularly free from the defects mentioned 

 above, and with the sources of illumination used the sun and the electric arc the prolongation 

 of the exposure was not sufficient to cause inconvenience from casual vibrations. All development 

 was carried on with a single solution hydrokinon developer, composed of hydrokinon, 5 gr., 

 sulphite of soda 20 gr., carbonate of potash 15 gr., to the ounce of water. When fully developed 

 the plates were fixed in a solution of sodium thiosulphite, rinsed thoroughly and placed in a 

 saturated solution of alum for five or ten minutes, washed in running water from one to two 

 hours and allowed to dry spontaneously. The plates were not retouched in any manner, as it 

 was most desirable that the finished prints should be entirely photographic and should be as 

 little affected by the personal equation of the operator as possible ; it was for this reason also that 

 the artotype process of Mr. Bierstadt was chosen for the reproductions. This is the well-known 

 gelatine process which is used under various names in this country and in Europe, and may be 

 briefly described as follows : The negative is placed in contact with a glass plate coated with 

 gelatine containing- bichromate of potash (bichromated gelatine) and then exposed to light, as in 

 ordinary photographic printing. Gelatine is hygroscopic and unaffected by light, but bichromated 

 gelatine is rendered insoluble and incapable of absorbing moisture by the action of light in direct 

 proportion to the length of time the light action continues. The light passing through the 

 transparent portions of the negative renders the bichromated gelatine beneath non-absorbent, the 

 partially opaque parts of the negative allow a varying light action according to the degree of 

 density of the silver deposit, and the opaque parts prevent any action of light on the bichro- 

 mated film ; as a result, therefore, the negative is reproduced in all its details of light and 

 shadow by the varying power of absorption of the bichromated gelatine. The action of the 

 light having continued for the proper length of time, the plate is removed from beneath the 

 negative and placed in cold water, in the dark, until all the bichromate of potash has been 

 dissolved out ; it is then allowed to dry and is ready for the press. The plate is then firmly 

 fixed in a horizontal position in a common lithographic printing-press, with the gelatine coating 

 uppermost, and saturated with water, which is absorbed by the gelatine, as stated above, in a 

 degree varying inversely as has been the amount of action of the light ; the surplus water is 

 then removed and a roller charged with printer's ink is rolled over it in all directions. The 

 ink used is a greasy ink and will adhere to the non-absorbent portions of the plate, but will 

 be repelled by the moist portions either wholly or partially, according to the amount of moisture 

 they retain. When sufficiently inked, a sheet of paper is laid upon the plate and receives the 

 pressure, as in ordinary printing, the ink is transferred from the plate to the paper, and the 

 result is a finished print. 



