20 Transactions of the Society. 



at all reaches the left hand of the lens only and from the right 

 hand side of X reaches the right hand side only. If the light from 

 the lens is geometrically divided and passed to one eye at A and 

 the other at B, a perfect stereoscopic picture will result, as though 

 the eyes were looking on both sides of a card held in front of them 

 in the well known experiment on binocular vision. It must, how- 

 aver, be remembered that a Microscope inverts the image, and con- 

 sequently to pass the correct image to the eyes to obtain the 

 stereoscopic relief the two beams must be crossed over as in 

 Form 4 the Naehet, and Form 5 the Wenham, or else the images 

 must be re-erected as in Form 3. 



5. The images are viewed in Form 1, Type I, with the two 

 eyes parallel, in the other forms of this type with the eyes con- 

 verging to a greater or less degree. 



Properties of the Second Type of Binocular Microscope, which 

 has a Physically Divided Beam. 



1. TJce resolution of this type is in all cases equal to that of a 

 monocular provided the surfaces of the prism are perfect, because 

 each eye receives a full size beam of light. 



2. The prisms may be placed in any position in the beam of 

 light between the object glass and eye-piece, and need not be 

 placed close to the back lens. 



3. The relative illumination in the two eyes in No. 6 Powell 

 and Lealand, No. 7 Wenham Powell, No. 8 Abbe, is very unequal. 

 In the Leitz and my own form it is equal in the two eyes, and no 

 special care is required as to the equal illumination of the two 

 halves of the object glass. Even if a single beam of oblique light 

 be used for resolving a diatom, which enters the object glass from 

 only one side, the second type of binocular is as efficient as the 

 monocular. In type 6, Powell and Lealand, the light when it 

 reaches the first face of prism 1 is divided into a reflected beam 

 and a transmitted beam, the latter being about five times as bril- 

 liant of that of the other. Wenham, in his modification of the 

 above, by placing his reflecting surface at a greater angle to the 

 incident light, increased the brilliance of the reflected light, but 

 even then the relative intensity differed from about 1 to 3. Abbe 

 adopted Wenham's modification in this respect ; he claims that an 

 unequal illumination is advantageous for those microscopists who, 

 before taking to the use of a binocular Microscope, have already 

 reduced the sensitiveness of one eye with a monocular instrument. 

 This argument is one that is difficult to follow, for if the binocular 

 Microscope should come into general use, which, now that the 

 correct type of instrument is being designed is extremely probable, 

 it is important that it should be constructed for the normal 



