82 ELEMENTARY PHOTO-MICROGRAPHY. 



negatives of diatoms 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 \ ounce. 



Citric acid J 



Water . . . . . . 10 ounces. 



The stain is not always a disadvantage, for it 

 may actually improve the printing capacity of the 

 negative. After the clearing process the plate 

 must be again washed. 



Small holes or clear spots are caused by dust, 

 and larger spots probably by air bubbles in the 

 developer. These may be painted out with a small 

 brush charged with colour. 



When the negative is too thin it may be altered 

 by intensification, and its printing quality greatly 

 improved. 



But intensification is only recommended when 

 the trouble of taking a fresh negative is too great. 

 The photo-micrographer is in a different position 

 from the landscape photographer, who cannot 

 easily revisit the scene of his subject under the 

 same conditions of light and weather, whereas the 

 microscopist can reproduce it at pleasure. 



Intensification should only be regarded as a 



