The Binocular Microscope of the Past, and a New Form. 21 



observer whose eyes have not been previously damaged by the use 

 of a monocular instrument. It is possible to suit special cases, in 

 either the Leitz or my own new model, by constructing instruments 

 in which the brilliance of the respective eyes is varied by varying 

 the amount of the reflecting silver deposit, but it is probably not 

 desirable. 



4. Stereoscopic, Belief, — Dr. Jentzsch says that the Abbe binoc- 

 ular eye-piece threw all previous models into the shade, though he 

 states also that it had a very restricted use. Probably in Germany, 

 where the Wenham Binocular was not popular, this is accurate, 

 but in this country and also in America no instrument has yet 

 thrown the Wenham binocular into the shade, and it may be said 

 to be the only binocular instrument that has hitherto been made 

 in large quantities. The Abbe eye-piece is a difficult instrument to 

 keep in adjustment, and has various disadvantages, but it was of 

 great interest from a scientific point of view on account of the 

 controversy that it occasioned as to whether it gave a stereoscopic 

 picture. To elucidate this problem, Abbe pointed out that the 

 Eamsden circle of a Microscope is the conjugate image of the 

 aperture of the object glass, and that if the beam of light entering 

 the eye were divided at the Eamsden circle instead of at the aper- 

 ture of the object glass, exactly the same rays would be excluded 

 from each eye, and the optical effect would be the same. Suppose in 

 Plate II d represents the object glass and b c the eye-piece of the 

 Microscope, z the Eamsden disk, which is the conjugate image, i.e. 

 a small picture of the lens d, a shutter cutting off half the lens D 

 or a smaller shutter cutting half its image at z allows just the 

 same rays to enter the eye. Thus Abbe proved that a stereoscopic 

 effect can also be obtained with any of the second type instruments, 

 provided a diaphragm be placed in the Eamsden circle of each eye- 

 piece which cuts out half the rays. It could be turned into a 

 pseudoscopic effect by cutting out the wrong half of the rays or no 

 stereoscopic effect at all by making use of the whole aperture. If 

 by means of a lens held above the eye-pieces the Eamsden disks of 

 a Wenham binocular Microscope be examined, they will be found to 

 be half discs in each eye, small pictures of the back of the bisected 

 object glass, whereas in binocular Microscopes of the second and 

 third type the Eamsden circle is a complete disk. Abbe pointed 

 out that if in these Microscopes that give a complete disk, a D- 

 shaped diaphragm be placed over the Eamsden circle, the same 

 stereoscopic result would be obtained as in the so-called stereoscopic 

 binoculars, except that half the light would be wasted. Theoretic- 

 ally this is quite correct, but there is a serious practical drawback. 

 The proper use of the Microscope necessitates that the observer's 

 eye should be placed so that the Eamsden disk is inside the eye 

 very close to the pupil, and the diaphragm made by Abbe cannot 

 therefore be placed in the right position. In consequence of the 



