374 E. M. NELSON ON BINOCULAR MICROSCOPES. 



A great deal has been made of the difference between parallel 

 and converging tubes. It has been urged that parallel tubes are 

 conducive of eye strain and fatigue. Having now had a Leitz 

 microscope in constant use for nearly three months, and having 

 done prolonged work with it, no more eye-strain has been found 

 with the parallel tubes than with a Wenham, and with both 

 there is less fatigue than with a monocular. 



To me the image plane in a microscope appears at so definite 

 a distance that I seem able to hold a pencil in front of it, or 

 behind it, or touching it. When using a binocular I simply look 

 at the image in this plane, being quite as unconscious of either 

 the parallelism or convergence of the eyes as if I were looking at 

 various objects in the room, or on the table. During the course 

 of these experiments several curious observations were made. 

 Various persons were asked to examine the images in the Wenham 

 and in the Leitz for the purpose of ascertaining their opinion as 



W M 



Fig. 5, 



to the relative amount of stereoscopic effect in each. Two persons 

 having good normal vision saw no stereoscopic effect in either, 

 the images in both instruments appearing quite flat ; one of them 

 could see no stereoscopic effect either in an ordinary stereoscope 

 or in a field glass. Two others saw stereoscopism in the Wenham, 

 but not in the Leitz with the Mercer method. With the same 

 object and same power in both (| inch and B eye-piece), most 

 persons said that stereoscopism was stronger in the Wenham, 

 owing probably to want of practice and experience with the 

 Mercer method. 



When the inter-ocular distance in the new binocular is kept 

 of the same width as the inter-pupillary, the microscope is a 

 non-stereoscopic binocular. The Mercer plan of reducing the 

 inter-ocular distance is found to produce fatigue on account of 

 the flickering of the image when the Ramsden disc is small. 



Figure 5 shows the reason why eye strain and fatigue, 

 which are present with the Mercer method, are absent with 



