134 DEFECTS IN NEGATIVES. 



disturb the corrections of the object glass, so that a 

 weak negative is the result. 



IV. Non-actinic colour of medium. This and the 

 former fault, are especially noticeable with balsam. 

 Diffusion of the colouring matter of the object, 

 particularly if red, through the medium may also 

 weaken the image. 



V. Delicate structure of the object produces a 

 weak image. This may be obviated by staining the 

 specimen with carmine, which cuts off most of the 

 actinic rays, and if cobalt blue glass is placed be- 

 tween the source of illumination and the object, it 

 stops off the remainder. The resulting negative will 

 be more distinct than the visual one. 



VI. Specimens stained with logwood or blue pig- 

 ments are unsuitable for micro-photography. 



Objects of a yellow colour or great delicacy give 

 the best results with wet plates. Those of a red 

 colour or coarse structure the best with dry. 



With different kinds of development a plate will 

 give different results. For instance, where it is 

 necessary to reproduce a great variety of shades, 

 alkaline pyro development is superior to ferrous oxa- 

 late, with dry plates. No intensification should be 

 attempted with wet plates, other than that obtained 

 by ordinary development. This is especially the case, 

 when the negative is required for enlargement. 



For direct printing, a thin negative obtained with 

 a wet plate, sometimes gives better gradations when 

 subsequently intensified with silver. A dry plate, 

 if the ferrous oxalate development is stopped before 



