The Binocular Microscope of the Past, and a New Form. 19 



Properties of the First Type of Binocular Microscope, which has a 

 geometrically bisected beam, one half going to each eye. 



1. Resolution. — As compared to monocular vision this type is 

 not equal to a monocular, because the diffraction image of a point 

 is increased in consequence of the aperture of the light forming 

 each individual image being half that of the object-glass, and each 

 image has less detail in it on this account. The fact that the two 

 images are combined in the brain does not fill in the detail when 

 it has once been lost in the images in consequence of the half- 

 size bundles of light. 



2. The geometrical bisection of the beam must take place near 

 the back nodal or equivalent plane. Dr. Jentzsch says it should 

 take place at the back focal plane of the object-glass, but this is a 

 mistake. It is even worse, as it should be at the back equivalent 

 plane in order to ensure a proper sub-division of the rays, which 

 come from the right-hand and left-hand sides of an object-point 

 respectively, otherwise these rays have become mixed up and 

 cannot be sub-divided except at the Kamsden disc outside the 

 eye-pieces, which will be referred to later. For this reason this 

 first type of instrument can only be used with low-power lenses, 

 unless the high powers are specially mounted exceedingly close 

 to the bisecting prism ; even when this is done great care must be 

 taken with the illumination to ensure equal lighting to both 

 halves of the object-glass. I am exhibiting to-ni^ht a binocular 

 form of the well-known Wenham make, with a y 1 ^ oil-immersion 

 mounted specially short showing tubercle bacilli, and you will 

 noti-je that the performance is extremely satisfactory and the 

 stereoscopic relief very marked, but the resolution is not equal to 

 that of a monocular instrument. The change from binocular to 

 monocular vision in this form by simply pushing the prism out 

 of place is so quick and simple that I do not think the lack of 

 resolution has been the only reason why this form has not more 

 generally been used for high-power work. 



3. The illumination in each tube in this type is similar in brilli- 

 ance if the illumination is such that the two halves of the object 

 glass receive equal amounts of light. 



4. Stereoscopic Relief. — This type of instrument, excepting forms 

 1 and 2 which give pseudoscopic images, gives a stereoscopic relief 

 of a very marked kind, indeed, of what is strictly a somewhat 

 exaggerated character. This fact has at times been disputed, it has 

 been stated that the stereoscopic effect is purely an illusion. The 

 small diagram (Plate II) renders the reason for the stereoscopic relief 

 clear. Suppose that represents the objective, and that an object 

 at X consists of a fine blade of material placed on end, all the 

 light from the left hand of this blade which enters the object glass 



