132 DEFECTS IN NEGATIVES. 



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 

 cle'ir 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. 



Eeflection 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 micro- photo- 

 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 r< quiivd in ordinary photography. All 

 beginn rs as a rule tall into the error of supposing 

 that the field surrounding the bjeci usi b opaque. 



convej .. oi i e appe ranee oi microscopic objects by 



engraviugs. All micro-photographs should be trans- 





