ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1061 



action of the luminous rays proportional to the " photovaleur " of the 

 different whites of the image it traverses. The preparation of the 

 glass plates requires the successive employment of the following 

 products : — 



Mixture A. Whites of two eggs beaten to a froth ; solution of 

 silicate of soda, 60 parts ; distilled water, 120 parts. This solution 

 having been introduced into a fiorentine flask is allowed to stand for 

 twelve hours. It is the lower limpid layer which is poured on the 

 glass plates. 



Mixture B. Very hard white gelatin, 20 parts ; distilled water, 

 200 parts. To this solution, made in a water-bath at a temperature of 

 45°-50° C, is added bichromate of ammonia 4 parts, dissolved in 

 distilled water, 40 parts ; ammonia, 15 drops. The mixture is filtered 

 at a temperature of about 45° through white filter paper. 



In order to obtain a uniform layer, the glass is placed on three 

 wooden cylinders, provided with adjusting screws ; and then some of 

 the mixture having been poured on, the glass slide is gently tilted 

 with the hand, and then a white thread rendered tense by being 

 stretched from the points of an iron arc, serves to carry along the 

 viscid fluid without loss of continuity and production of bubbles. 

 The glass is next dried in the air, and afterwards washed in cold 

 water for five minutes. It is next placed in a heating apparatus and 

 carefully levelled in the horizontal position. Here it is left for two 

 hours at a temperature of 57° C, and while still warm it is coated 

 with a layer of mixture B, the same procedure being adopted as for 

 mixture A. The glass is then again transferred to the heating stove, 

 where it is allowed to dry thoroughly at a temperature of 57° C. 

 (about two hours). 



When dry, the glass is sensitive to luminous rays. After cooling 

 it is placed in the press-frame above and in contact with the negative, 

 care being taken to cover all parts of the glass which are not to 

 receive the luminous action with black paper. Exposure is then 

 made to diffuse light. The duration of the exposure varies from one 

 hour to five minutes, according to the greater or less intensity of the 

 negative. The image is shown in relief on the layer of bichromated 

 gelatin. 



Before proceeding to print, the glass must be well washed to 

 remove any excess of bichromate, or better, it may be left for five or 

 six hours in running water. It is next dried in a dark place. To 

 obtain the positive the glass is moistened with the following mixture : 

 Glycerin, 100 parts ; ammonia, 5 parts ; hyposulphite of soda, 2 J 

 parts. After ten minutes the excess of the " Moistener " is removed, 

 first with a sponge, and next with a piece of clean linen ; the latter 

 must be dabbed, and not rubbed on. 



The plate is to be inked with two special phototypic inks, laid on 

 with rollers. It is necessary to remoisten with the glycerin mixture 

 after every dozen copies. From four to eight hundred copies may be 

 obtained from one plate. 



It is not necessary to perform the operations in an absolutely dark 

 room ; it is almost sufficient to draw down the blinds. 



