168 PRACTICAL PHOTO-MICROGRAPHY. 



glasses may doubtless be used, but this is the one most likely 

 to be within reach of every microscopist. 



The other salient difficulty with insects is a photographic 

 one and depends on color. Insects are frequently very dense 

 yellow in color which may be overcome by use of color-correct 

 plates with or without yellow illumination ; but when in addition 

 to a densely non-actinic body they have pellucid wings, antennae 

 or legs, the difficulty becomes very great indeed, and skilful is 

 he who can surmount such a concatenation of difficulties. 

 Again the worker must look to careful and intelligent use of 

 color-correct plates and colored screens or light. Exposure 

 must be full for the densest part of the object ; development 

 should be sharp and short, and as soon as details are out in the 

 dense parts, the negative should be fixed and intensification 

 resorted to. We have an idea, the outcome of some experi- 

 ments, that for this class of work the hydroquinone developer 

 with the caustic alkalies soda or potash is eminently suited. 

 (See page 96.) 



Pellucid Objects, such as some diatoms, present difficulties 

 of their own which we must notice. The difficulty is partly 

 in the optical, partly in the photographic department of our 

 work. By lowering the angular aperture of our lens we may 

 produce more visual contrast between the pellucid object and 

 the ground on which we see it, but we at once lose definition 

 and resolution. This holds good in the photographic as well 

 as ocular branch. If we lower the force of our radiant-power 

 we improve matters visually by taxing less the " accommoda- 

 ting" power of our eye, but the same step has no advantage 

 in our photography, for lowering the light simply entails in- 

 creased exposure. With objects of this pellucid nature our 

 best plan is to cut down the angle of our condenser as much as 

 we can without any loss of definition or resolution, (which is 

 equivalent to using as much as necessary of our objective's 

 available aperture and no more), keeping the exposure as short 

 as is consistent with getting a black negative ground, using a 

 thickly coated plate, an emulsion replete with silver haloid, 

 and a slow system of development, preferably perhaps hydro- 

 quinone with sodic or potassic hydrate ("caustic" soda or 



