144- DEFECTS IN NEGATIVES. 



generally caused by the plate coming in contact with 

 the fingers, or some unclean surface, and in the 

 collodion process insufficient drainage before placing 

 it in the carrier. 



Frilling is very liable to occur in hot weather with 

 gelatino-bromide plates. It commences by a blister- 

 ing of the film at the edges, and its extension may 

 be prevented by immediately immersing the plate in 

 a saturated solution of alum. This also serves to 

 clear the negative. 



Blurring of the image may be due to a variety of 

 causes. The principal are tremor of the apparatus 

 during exposure, and reflection from the back of the 

 plate. A black velvet backing similar to that already 

 described will entirely prevent the occurrence of the 

 latter. 



Reflection from the surfaces of the crystals sus- 

 pended in the film has been mentioned, and dying 

 the film suggested as a remedy. This lessens the 

 rapidity too much to be available in photo-micro- 

 graphy. 



It is well known that the invisible part of the 

 spectrum acts on bromo-iodised plates, and from the 

 results obtained with mono-chromatic light, we are 

 inclined to refer a blurred condition of the image to 

 the unequal refrangibility of the different parts of 

 the spectrum. It is for this reason also that the em- 

 ployment of cobalt blue glass increases the distinct- 

 ness of the image. 



The character of the negative differs considerably 

 from that required in ordinary photography. All 



