218 



PHOTOMICROGRAPHY 



plate moved along a little after each. Half the plate 

 was coated on the back with a thin film of caramel. The 

 print shows the spreading of light described in the last 

 section as limiting the resolving power of the plate, and 

 indicates that it is least with a short exposure, and 

 gradually increases. The secondary images, caused by 





Fig. 68 



HALATION 



light which has passed from the brightly lighted lines on 

 the film to the back of the plate and was there totally 

 reflected, are seen to occur only when gross over- 

 exposure has been given. As has been pointed out, 

 over-exposure is liable to occur in parts of a photo- 

 micrograph, and it is advisable to take such preventive 

 measures as will avoid the reflections. They are not 

 apparent on the backed half of the negative, showing 

 that the coating of caramel is an effective preventive, 

 but it is obvious on examining the figure that the backing 



