136 PHOTO-MICROGEAPHY 



time previous to taking the pliotograph ; in fact, to do it at one's leisure, noting the 

 place by the Verniers attached to the mechanical stage. By this roeans, when the 

 actual photograph is to be taken, the Verniers have only to be reset, and no search 

 made at the time. 



We say this because searching for an object when the microscope is horizontally 

 placed on the table necessitates a continued bending of the body over it, which is cer- 

 tainly fatiguing. This is a fault we are aware of in OQr own arrangement, about 

 w^hich we have spoken, and it is remedied to a degree in the best forms of expensive 

 apparatus sold ready made, by having a sort of turntable to the microscope, which 

 allows the whole instrument, with its jet and condensers, to rotate away from the 

 camera, and permits the operator to hunt through the specimen certainly with more 

 comfort than is afforded by the use of our own plan. This we wilHngly admit, but 

 what we personally have found is that it is far better, as previously pointed out, at 

 some other time to have searched through the slide, and not to leave it to the moment 

 of taking the picture. Hence the loss of this arrangement, which adds sensibly to the 

 expense, is more apparent than real. 



Let us select a Navicula lyra. Fig. ii, Plate III., and, having oiled the immer- 

 sion objective to the cover-glass and obtained critical light, we may for the specimen 

 under consideration close the ii is diaphragm a little, taking especial care to avoid 

 diffraction effects. Tlie attention of the photographer must be very close, or they 

 may be very easily overlooked. The auxiliary lens had better be used, and perhaps 

 the faintest amount of obhque light, to " show up " up the dots. It is not an easy 

 object to take, as the diatom itself is not flat, being as its name imphes — boat-shaped. 

 Occasionally monochromatic light — green is very good — may be employed. The 

 photograph shown in Fig. ii, Plate III., should be compared with Fig. lo. The 

 better definition in the former is due to the superior N. A. of the ^ over the |- inch. 



It may require half to a minute's exposure with the pot-green, but with Gifford's 

 F lens screen a trifle shorter time—although considerably less if without. The exact 

 time is difficult to state, for with these high magnifications the state of the lens, its 

 quality, and the purity of the gases used, not to mention the blending of the two 

 gases at their best illuminating power, are all such variable and yet such important 

 factors. Of these one should be especially mentioned, it is the purity of both gases. 

 The hmes, it is true, are difficult to get good, but when a good tin is opened, and if 

 kept well shut down when not in use, all of its contents are mostly of the same 

 quality. But with the gases it is different. The amateur cannot test either Mdthout 

 elaborate means, not often at his disposal. Hence he may sometimes find that his 



