372 E. M. NELSON ON BINOCULAR MICROSCOPES. 



as to the equality of the results, the ortho-stereoscopism in 

 the Wenham being superior to that in the other. 



In the Wenham and Stephenson, ortho-stereoscopism is weak 

 with objectives which have less than 20 of angular aperture (say 

 1^ inch of *17 N.A.), and the divided objective breaks down with 

 high powers. A divided objective binocular may be said to be at 

 its best with a \ inch ; good with 1 inch, |, T 4 (j-, and g ; fair with \ ; 

 but failing with a 4. Very small Wenham prisms have been 

 made and mounted on a funnel and placed in the mounts of a 

 Y2- ', the result being so indifferent that further experiments 

 in that direction were abandoned. 



The Wenham plan possesses a great advantage over all other 

 kinds of stereoscopic binoculars, viz. that the straight tube is 

 free from glasses, prisms, or other appliances likely to disturb the 

 image. You will naturally ask, Why then was -the Powell non- 

 stereoscopic system introduced ? The answer is that it was 

 intended to come in where the W'enham left off, for Powell 

 engraved on his Wenham prism, " For Low Powers," and on his 

 own prism, " For High Powers." The reason why the high-power 

 prism fell into disuse was on account of the poor definition that 

 could be obtained with it. It bad no clear tube like the Wenham, 

 and it should be remembered that prisms and flat glass surfaces, 

 owing to the manufacture of prism field-glasses, are now made 

 with a precision and accuracy altogether unknown in 1865, when 

 Powell made his. 



Binoculars of the Wenham or divided lens type have the dis- 

 advantage of indifferent definition of objects placed vertically 

 in the field. If, for example, that well-known test for medium 

 powers, the hair of the Polyxenus lagurus, be placed vertically in 

 the Wenham, with, say, a one-third objective, the definition 

 will be fuzzy ; but directly the hair is placed horizontally in the 

 field, the image becomes sharp. In ordinary work with a 

 Wenham, where an ortho-stereoscopic image is of primary im- 

 portance, this defect is not noticed, and probably only a few 

 raicroscopists are acquainted with it. But with the Powell type 

 of binocular, this error does not exist. The image is the same 

 in all azimuths. Now, in the Wenham high-power binocular, 

 which was introduced in reply to Powell's, the beam was divided 

 by two right-angled prisms with an air-space between tbem, the 

 inclination of the surfaces being adjusted near to the critical 



