146 . DEFECTS IN NEGATIVES. 



I. A weak bath solution, or too strong a developer 

 with wet plates. 



II. Over exposure, or unsuitable condition of de- 

 veloper with dry plates. 



III. High refractive index of the medium in which 

 the object is mounted. This prevents proper diffe- 

 rentiation of structure both visually and chemically. 

 Even when it does not do so visually, it seems to 

 disturb the corrections of the object glass, so that a 

 weak negative is the result. 



IY. 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. 



YI. Specimens unless stained very deeply with 

 logwood or blue pigments are unsuitable for photo- 

 micrography. 



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,, 



