Ch. Ill] BINOCULAR MICROSCOPES 89 



occur only when the oculars are spread the right amount to bring the 

 eye-points the same distance apart as the pupils of the eyes of the 

 observer, and the eyes are at the correct level. 



Hold the head close to the oculars and look into the microscope. 

 Focus as usual and the image will be satisfactory. Now to tell whether 

 the image is seen with one eye or with both, hold the head still and shut 

 the eyes alternately. If only one eye is being used no image at all 

 will be seen when that eye is closed, but when the other is closed there 

 will be no change in the appearance (§ 147 a). 



If it is found that only one eye is being used, change the spread of 

 the oculars by grasping the prism holder or drums or the tubes above 

 these with the two hands and increase and diminish the distance be- 

 tween the tubes until both eyes are receiving the light, and there is 

 an image in each eye. When this occurs and one once gets the stereo- 

 scopic effect there will never be any doubt in the future whether the 

 vision is monocular or binocular. 



§ 147a. In some makes of binocular microscopes (the Spencer Lens Co.'s, 

 for example), there is a little shutter just above the objectives which can be 

 turned to either side, covering the back of the corresponding objective. If 

 the image is still apparent whichever objective is covered then of course both 

 eyes are seeing th'e image, but if the image is wholly obliterated when the shut- 

 ter is on one side, that is the only side giving an image, and the tubes must be 

 changed in position to get the correct pupillary distance of the eye-points. 



§ 148. Focusing if the eyes are unlike. — It occasionally happens 

 that the eyes of the observer are markedly different. Provision is 

 made for focusing one tube or one objective to compensate for this. 

 If it is necessary to make this special adjustment, focus first with the 

 rack and pinion and get the focus as sharp as possible for the tube 

 having no special adjustment; then, without changing the general 

 focus, turn the milled ring of the other tube until the image for the 

 corresponding eye is also perfectly sharp. If now one uses both eyes 

 the images should be equally sharp and the binocular vision good. 



Of course the lower the objective the less need there is for special 

 adjustment. 



§ 149. Opaque objects for the double-objective binocular. — Put 

 a piece of black paper or velvet on the stage, and upon that a piece 



