477 



differences between the process of Lumiere and those which 

 preceded it were briefly explained, great emphasis being laid 

 upon the importance of keeping to one process, it being of little 

 use to employ a set of screens of one kind with plates which had 

 been prepared for another. The screen and the plate must be 

 adjusted to each other in order to produce any really good result. 

 Coarse objects might l)e done in the former way, but for the 

 reproduction of any delicate tints strict attention must be given 

 to the relation betw^een the screens and the plates. The negatives 

 were not taken upon the ordinary films, but upon thin plates of 

 mica, which were coated with gelatine, treated with bichromate of 

 potash, and also with bromide of silver, the object of which was to 

 enable the photographer to see what he had done. The image was 

 formed on the bichromate-gelatine in from five to ten minutes ; 

 the plate was then put into hot water, which washed away all 

 those portions of the gelatine which had not been acted upon by 

 the light. The prints were then stained in the primary tints, 

 and on being superposed, the result was a picture in the complete 

 colours of the original object. Some care was of course required 

 in staining, so that one colour did not overpower the others ; but 

 if the print was found to be too strongly stained, it could be easily 

 washed out in a weak solution of glue, and if not stained enough 

 it could as easily be returned to the staining bath for as long as 

 might be necessary. Mica had a very great advantage over 

 celluloid in being perfectly clear, so that the light passed through 

 the three films as easily as through one, whilst the thinnest 

 celluloid always intercepted some light ; then the mica films, being 

 thinner, could be laid closer together than celluloid. Mica had the 

 further very great advantage that it was not injured by heat, 

 which would completely destroy any film of celluloid. The best 

 guide in developing was to note any white object in the picture, 

 and if this was found to be correct, all the colours would be 

 correct also. 



The subject was then illustrated by a number of slides shown 

 on the screen : the first, taken from a well-known coloured picture, 

 was shown separately on the red, the green, and the blue films, 

 which, w^hen placed together, combined to form a picture of 

 great brilliancy and beauty, with perfect fidelity to the original. 

 Other slides of flowers, china, etc., were followed by microscopic 

 objects, such as blood-corpuscles, stained sections, and bacteria, 



