PROCEEDINGS OF THE SOCIETY. W 



effects, said that no doubt when the Ranisden circle was small, as was 

 the case with high-power eye-pieces, more careful placing of the eves 

 was necessary, but it was quite simple to get full stereoscopic effect, 

 even with an almost microscopic Ranisden circle. He thought this 

 might be attributed to the fact that it was by no means necessary to 

 cut off the entire half of the rays to get stereoscopic effects ; it was only 

 approximation to the bisection of the rays that was required, and quite 

 as much as was necessary for differentiation of depth could be obtained. 

 With a high-power objective the stereoscopic depth was exaggerated, 

 and so the Ramsden circle could be cut down to a slight extent. Where 

 there were two isolated objects, one behind the other like grains of sand, 

 or diatoms, it was an extraordinarily interesting sight to see these cross 

 each other. 



With reference to Mr. Cheshire's remarks about the Ramsden disk, 

 lie knew that it was sometimes said that the latter was the conjugate 

 image of the back focal plane of the object-glass. This was a wrong 

 definition. The Ramsden disk was the smallest aperture after leaving 

 the eye-piece, and was the conjugate image of the back equivalent plane, 

 or, speaking roughly, of the aperture of the object-glass. With a low- 

 power object-glass the image of the back focal plane was about an inch 

 or two behind the eye-piece of the Microscope. If the back focal plane 

 of the object-glass were used for bisecting the rays for a binocular 

 Microscope, they had already somewhat intermingled at this plane, and 

 in most cases it would be impossible to make a division of this plane. 



In reference to what had been said about correction being interfered 

 with by a large piece of glass, the whole point was that if light were 

 coming directly through a parallel plate of glass, the aberration was 

 practically nothing, but if at a great angle the effect was very serious. 

 The angle at which the light passed through the thick plate of glass at 

 the back in the case under discussion was very nearly parallel, a parallel 

 plate placed in front of a telescope, it had no result whatever, and the 

 effect of placing a block of glass behind the object-glass was almost 

 the same as in the telescope, whereas even a very thin plate placed 

 between the object-glass and the object where the angle of the light was 

 very great produced a large aberration. 



There had been a good deal of discussion as to the absorption of 

 light by the glass : if it was first-class optical glass the absorption was 

 so small as to be considerably less than 1 p.c The light absorbed by 

 an 8-in. piece of the best optical glass was less than 1 p.c. Light 

 passing through a thick piece of best optical glass lost practically 

 nothing by absorption, though it lost considerably by reflection at every 

 surface, generally from 4 to 5 p.c. at each surface. 



In regard to what had been said as to the alteration in the tube- 

 length upsetting the correction of the objective, the standard tube-length 

 was when the eye-pieces were in their mean interocular position, and as 

 the alteration in the interocular distance was produced very rapidly the 

 error in the tube-length caused by setting this adjustment was not 

 sufficient to interfere with the correction of the object-glass. If 

 observers desired the amount could be exactly counterbalanced by an 

 alteration in the length of the nose-piece. The ordinary monocular 



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