114 THE MICROSCOPE 



readies 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. A microscope inverts the image, and 

 consequently to pass the correct image to the eyes to 

 obtain the stereoscopic relief, the light from the right- 

 hand side of the object glass must be passed to the 

 left eye, and vice versa. 



The first kind of binocular microscope described 

 (Fig. 99) bisected the beam of light at the back of the object glass 

 and passed one beam to each eye, and for long it was supposed 

 that unless the beam were thus divided immediately behind the 

 back lens of the object glass, no microscope could be made which 

 would give stereoscopic relief. By examining the diagram of the 

 rays passing through a microscope as indicated in Fig. 102, it will 

 be seen that the rays of light intermingle after they leave 

 the object glass, and at no other place between the lenses could 

 the right-hand half of the rays entering the object glass be 

 separated from the left half. It might be done for any particular 

 bundle like that indicated by the shaded portion, but not for all 

 such bundles ; a diaphragm placed, for instance, over half the 

 field half-way up the tube would obliterate almost all the light 

 from one side of the object, and allow all to pass from the other 

 side of the object. It would not obliterate all the rays that 

 enter from one side of the object glass, but would obscure half 

 the object. 



It will, however, be noticed in Fig. 102 that all the rays of 

 light, after passing through the microscope, pass through a small 

 area called the Kamsden circle [zz') just above the eyepiece. 

 This circular disc is a picture formed by the eyepiece of the aper- 

 ture of the object glass. At this place the light may be divided 

 just as if it were the back of the object glass, and if in this place 

 a complete circular bundle of light is received from each eyepiece 

 of a binocular microscope it is possible, by placing suitable dia- 

 phragms at these points, to exclude from the right eye all light 

 that enters the object glass from the right-hand side of each 

 point on the object, and from the left-hand eye all light that 

 enters the object glass from the left-hand side of the object 

 points. Thus, two D-shaped diaphragms placed at the positions 

 of the Ramsden circles exclude from each eye the correct portions 

 of light and give the stereoscope relief with the same efficiency as 

 the first kind of binocular microscope, except for the loss of light. 

 There is, however, a practical objection to this procedure. The 

 proper use of the microscope is dependent on the eyes being so 

 placed that these discs are within the eye very near to the pupils, 



