56 THE A B C OF PHOTO MICROGRAPHY 



ticulars. These may be mounted in duplicate 

 frames of the regular screen, or the ground 

 glass of the latter can be fitted so as to be 

 removable and replaced by the others; the 

 former plan being the preferable one. To 

 make the gelatine surface-screen a dry plate 

 of suitable size should be exposed for a brief 

 period to white light and developed until 

 slightly grayed over, when it is to be fixed in 

 the ordinary way and thoroughly washed. It 

 may be well to treat two plates in this manner, 

 one of which is to be placed on a rack and 

 dried. The other should be bleached in the 

 mercurial solution used for intensification, 

 well washed and dried. We have thus two 

 screens with very fine surfaces, one of which 

 will show the image, but rather faintly, to the 

 naked eye, the other requiring the aid of a 

 focusing-glass, as does the plate-glass also. 

 Provided with these three screens in addition 

 to the ordinary one of ground-glass the photo- 

 micrographer is prepared to successfully meet 

 all focusing problems. In using the gelatine 

 screens that surface is, of course, presented 

 towards the projected image and the focusing- 

 glass is applied to the back as with the one of 

 ground-glass. This is mounted in such man- 

 ner as to permit being focused through the 



