226 PHOTOMICROGRAPHY 



give an adequate representation. The use of colour 

 screens for this purpose has been described in Chapter IX. 

 and their effect depends ultimately, to a considerable 

 extent, on exposure, but a great deal can be done in this 

 direction by adjusting the exposure and development. 

 Under-exposure, followed by a lengthened development, 

 especially if a hard-working developer is used, will increase 

 the contrast between light and dark parts of an object, but 

 probably at the expense of detail. A longer exposure and 

 shorter development will decrease the contrast, but give 

 the shadow detail. 



Errors in exposure can be detected in the negative by 

 the following appearances, provided the plate has re- 

 mained in the developer for the normal time and the 

 errors are not too great. 



Over-Exposure. The contrasts of the negative are 

 good, the detail is all there, but the negative is too dense 

 to print easily. The tendency when developing an over- 

 exposed plate by inspection is to withdraw it too soon, 

 or when the density appears correct. This leads to flat 

 negatives, often, but erroneously, looked upon as the 

 necessary result of over-exposure. 



Under-Exposure. The negative will be thin but of 

 good contrast. A hard negative giving a " soot and white- 

 wash " print lacking in detail may easily result from 

 under-exposure, owing to the tendency to keep the plate 

 in the developer long enough to force up detail in the 

 shadows, by which time the lighter parts may be hope- 

 lessly dense and have lost all detail. 



Improvement of Poor Negatives. When inconvenient 

 or impossible to repeat an exposure, it is often desirable 

 to improve the printing qualities of a negative. Re- 

 touching is, of course, out of the question for scientific 

 photographs, as alterations in detail might be introduced 

 thereby, but there are a few permissible expedients. 



Spotting is resorted to to cover pin-holes caused by dirt 



