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Transactions of the Society. 



side. Fig. 53 shows the effect of this stop. The principal rays 

 of the imaging pencils now all pass through the principal focus 

 of the lens, so that on the image side of the lens they are 

 parallel to one another, and to the axis of the lens. If now the 

 eyepiece-scale be adjusted, either within or without the plane E, 

 no error will be introduced, because the distance read off will be 

 that between the intersections of the principal rays with the plane 

 of the eyepiece-scale, which is a constant. 



The use of a small circular aperture in this way, however, is 

 sometimes open to objection, because it cuts down so seriously the 



Fig. 52. — A simple projecting lens. 



Fig. 53. — A telecentric system. 



amount of light passing through the lens for imaging purposes. 

 Further, since in practice no adjustment can be made with certainty, 

 the most reliable results are obtained by adjusting, as accurately 

 as possible, the stop in the focal plane, and the eye-piece scale in 

 the image plane. But the latter adjustment is very difficult with 

 Abbe's arrangement, because the angular aperture of the imaging 

 pencils is so small. The light and focusing difficulties can, how- 

 ever, be got rid of in a very simple way. To do this it is only 

 necessary to draw a horizontal line across the scale S at right 

 angles to the direction of the usual scale divisions, and to use with 

 it, in the plane of the principal focus F, not a circular aperture, 



