PRACTICAL PHOTOMICROGRAPHY. 103 



lac, or better with one of the good negative varnishes sold for 

 the purpose. The instructions as to heating the plate are to 

 be carefully followed. Plain collodion makes a very good and 

 usually sufficient protection for the film. What we have to 

 avoid is damp, and of course our negatives must not be 

 scratched. The only objection to varnish is that if appreciably 

 thick it may prevent absolute contact for contact printing, 

 perfect " sharpness " being important in photo-micrography. 



A good negative presents gradations from nearly opaque 

 highest lights, to clear but well marked shadow detail. The 

 shape of a candle name should be discernible with difficulty 

 through the very densest parts of the negative. There should 

 be no clear glass, but some detail everywhere. The higher 

 lights as well as the deeper shadows should show detail. These 

 remarks hold good for nearly all kinds of subject, but there 

 are exceptions. 



If in a negative we find dense lights in close . juxtaposition 

 with clear shadows the negative is underexposed, unless the 

 subject is for " black and white " rendering. 



If any part is absolutely opaque and (1) some parts are quite 

 clear, underexposure, or (2) all other parts too dense, overex- 

 posure, is the probable cause. If the negative is grey all over, 

 nothing in it approaching opacity overexposure, or under- 

 development. 



If the negative is clear nearly all over, underexposure is 

 certain to be the fault. 



Frilling and blistering are not often met with now-a-days. 

 If a plate frills before it goes into the fixing solution it is a bad 

 look out for that batch; frequently the carbonates if long 

 applied in the developer cause frilling or isolation of the whole 

 film; ammonia has not this tendency. If the blistering or 

 frilling appear on first washing after the hypo, a cure will 

 usually be found in soaking the plates straight from the hypo 

 in a tray of common salt and water, say one part to twenty. 

 Too strong hypo solution may cause blisters. 



Fog, which is easily recognized when seen, must be traced 

 to its source. If the parts of the plate protected during ex- 

 posure, as by the paper mask, or by the rebates or corners of 



