182 PRACTICAL PHOTO-MICROGEAPHY 



small size for high-power work. It is important to see that 

 these plate-carriers do not interfere with the register of the plate 

 in any way ; that is, that a smaller or larger plate when put into 

 the dark- slide should both be in exact register with the focussing- 

 screen of the camera. In general, these conditions are exactly 

 carried out by camera-makers, but it is advisable to make sure 

 that there is no difference in their position before work is 

 actually attempted. 



This may be done roughly by measuring with a rule the 

 distance of the focussing-screen, or of the sensitive surface of the 

 photographic plate, from any fixed position in the camera itself. 

 This is not such an efficient method as an optical one. An 

 ordinary photographic lens of wide aperture should be put on 

 to the camera front and some sharply defined object such as a 

 window frame, or even better a sheet of printed matter care- 

 fully f ocussed on the ground-glass focussing-screen. The camera 

 back should then be inserted, and, assuming that it is a double 

 back which will allow of one-half of the slide being turned back 

 and the plate in position being exposed to view, a piece of ground- 

 glass may be put into the dark- slide in the position usually 

 occupied by the sensitive plate. The image is then observed, 

 and should, under these conditions, be equally well defined 

 when viewed on either the proper focussing- screen or on the 

 ground-glass inserted in the dark-slide carrier. 



It must be admitted, however, that in photo-micrography 

 such perfect registration under general conditions does not 

 affect the final result to the same extent as in ordinary photo- 

 graphy. The angle of the beam is usually so narrow that is, 

 the rays approximate so closely to parallel rays that a small 

 difference in position between the dark-slide and the focussing- 

 screen does not have so great an effect. Since, however, the 

 camera may be used for varying classes of work, it is a point 

 well worth attending to at the start ; so that whatever may 

 be attempted no depreciation of the result need be anticipated 

 from such a source of error. 



If greater exactness is required, a piece of plain glass may be 

 substituted for the ground-glass, and the image f ocussed by a 

 good focussing- magnifier. This method is both easier and more 

 efficient than taking a succession of photographs to ascertain 

 the correctness of register of the dark-slide or its carriers, as any 



