ELEMENTARY PHOTO-MICROGRAPHY. 55 



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. 



Intensification is only recommended when the 

 trouble of Baking a fresh negative is too' great. The 

 photo-micrographer is in a different position from 

 the landscape photographer, who cannot easily re- 

 visit the scene of his subject under the same condi- 

 tions of light and weather, whereas the microscopist 

 can reproduce it at pleasure. 



Intensification should only be regarded as a make- 

 shift. Mercuric chloride is a deadly poison which 

 should be kept under lock and key, and not be 

 brought into contact with the fingers. After the 

 negative has been freed from hypo it may be placed 

 in the following solution until the film becomes 

 white : 



Mercury bichloride ... ^ ounce 



Hydrochloric acid ... ^ dram 



Water ... ... ... 10 ounces 



It is then thoroughly washed and redeveloped 

 with a weak solution of metol and sodium car- 

 bonate, or any ordinary developer, which will 

 darken the film. It is again well washed and dried. 



Over-dense negatives may be reduced in opacity 

 by immersion in equal parts of fresh fixing bath 



