54 ELEMENTARY PHOTO-MICROGRAPHY. 



on the surface of the plate before fixing, yet after 

 fixing will be quite thin. 



Opacity should be judged by holding the nega- 

 tive up to the light. When washed and dried the 

 negative may be varnished if it is likely that it will 

 be much used, otherwise varnishing is no>t necessary. 



A good method of preserving negatives, and at 

 the same time of providing a ready method of selec- 

 tion, is to put each one in an envelope with a con- 

 secutive number outside. These may b.e stored in 

 disused dry plate boxes, outside of which should be 

 boldly printed the numbers of the slides therein. 

 For example, the first box will be i to 12, the 

 second 13 to 24, and so on. 



A book index will facilitate reference. Each en- 

 velope may have also written upon it the full par- 

 ticulars of exposure, though this may be obtained 

 from the exposure book. This system may be ex- 

 tended still further bv devoting certain numbers to 

 certain classifications. Thus all negatives of dia- 

 toms may be put in one box, insects in another, 

 anatomical subjects in a third, and so on. The 

 advantage is so great and the trouble so little that 

 it is worth the doing. 



When the negative has been stained yellow, and 

 it is desired to remove this, it may be immersed in 

 a solution of citric acid and alum 



Alum y<2 ounce 



Citric acid ^ ounce 



Water ... ... 10 ounces 



