10 



scopic slides. This makes a small portion of the groniid glass trans- 

 parent, but with the focusing" glass or eyeiDiece shown in PL I at a 

 one can focus on this clear field more accurately than on the ground 

 glass. This device is very useful when enlarging small insects. I 

 alwaj'S like to have a life-size picture of the smaller insects to use 

 beside an enlargement instead of the usual liair line. One rarely 

 gains much by enlarging iiisects which are an inch or more in size. 



The proper length of exj)osure will have to be learned by experience, 

 as it depends upon so manj^ conditions, the rapidity of the plate, time 

 of day, amount of light, magnification, etc. I usually expose an iso- 

 chromatic medium plate about five seconds for a picture natural size 

 under the most favorable conditions of light in nry workshop. Sev- 

 eral minutes are often required Avhen considerable enlargements are 

 made. I use regular jDhotographic lenses as much as possible in 

 enlarging, for they seem to give more detail and greater depth. Insects 

 are usually photographed natural size or larger, and to do this well 

 requires time exposures of from two seconds to several minutes. I 

 can not get good detail and other necessaiy qualities with "snap 

 shots ; " I must have a second or two, even with instantaneous ijlates 

 and good light. 



I alwaj'S use as small a diaphragm as possible (usually a No. 128 

 stop with opening about 7 mm. in diameter) under existing conditions 

 of light, liability of objects to move, etc. In photographing large 

 insects like the beetles, especially when enlarging them, it is absolutely 

 necessary to use a small stoj) or some parts of the insect will appear 

 out of focus in the picture ; in other words, a small stop gives greater 

 depth to the picture. 



PLATES. 



I now use isochromatic medium plates for most of my work. There 

 is much in getting used to one brand of plates. I use some instan- 

 taneous plates for restless insects or outdoor w^ork. And I find that 

 for copying line-engravings or drawings plates made especially for 

 such work give the best results. 



COLOR SCREENS OR "RAY FILTERS." 



These are sometimes indispensable in photographing many-colored 

 insects like the butterflies and moths. I find, however, that in most 

 cases the isochromatic plates give good color values, but certain 

 colors, like tliose on the butterfly Fenisecai(irqin))ius, require a color 

 screen to bring them out well. I am too amateurish as yet to be able 

 to determine from looking at an insect whether I had best use the 

 screen. If I do not get a satisfactory^ negative without, then I put on 

 the color screen. It is practically out of the question to use these 

 screens. in photographing living insects, for they lengthen the expo- 

 sure from 15 to 18 times. I like the glass screens, adapted for special 

 brands of plates, better than the liquid raj' filters. 



