430 EDWARD M. NELSON ON 



A third binocular is the new Parallel tube; * it, like theWenham, 

 uses a single objective. Three kinds of vision can be obtained 

 with this instrument, viz. non-, pseudo-, or ortho-stereoscopic, 

 which is effected by adjusting the separation between the tubes 

 either equal to, greater, or less than that of the interpupillary 

 distance of the observer's eyes. In this microscope, which some 

 of you may have seen, the illumination in each tube is nearly the 

 same. When the tube separation is equal to that of the inter- 

 pupillary distance, the whole of the pencils emerging from the 

 eye-pieces pass uninterruptedly through the pupils of the eyes, 

 the image therefore has its maximum brightness, and is non- 

 stereoscopic. But when the tubes are closed to about 1 mm. less 

 than the interpupillary distance, the inner portion of the emergent 

 pencils are cut out by the iris of the eyes, the image becomes 

 ortho- stereoscopic and loses brightness. 



This reduction in the brilliancy of the image is a guide in setting 

 the tubes to their proper separation for ortho-stereoscopic images. 

 If we examine the same low- power object with a Greenough and 

 a Wenham, arranging matters so that the measured magnifying 

 povNTcrs are identical, we shall find the stereoscopic relief in the 

 image given by the Wenham is a good deal below that of our 

 standard ; as the powers increase, this deficiency becomes less. 

 The falling off in the relief of the image in a Wenham mth powers 

 ranging from 1 inch downwards is an important point scarcely 

 noticed by microscopists ; on the contrary, the Wenham together 

 with the Stephenson are regarded by them as instruments yielding 

 perfect ortho-stereoscopic images. Now, if attention be directed 

 to the apparent size of the images, that in the Wenham will be 

 noticeably larger than that in the Greenough, althougli the 

 measured powers are identical. The Parallel tube binocular was 

 now set up, and the powers increased to Greenough 46, Wenham, 

 44, Parallel tube, 42. The amount of relief in the Greenough and 

 Parallel tube was the same, and in each of them it was greater 

 than in the Wenham, but the apparent magnification was much 

 greater in the Parallel tube, next in the Wenham, and least of all 

 in the Greenough. 



♦ Felix Jentzach, Ph.D., "The Binocular Microscope," Journ. 

 R.M.S., 11H4, pp. 1-16. 



