THE MICROSCOPE STAND 113 



and tiring to constantly change the direction of his convergence, 

 such is the force of habit that the mere action of bending the head 

 downwards induces the convergence of the eyes necessary for 

 examining near objects. 



Any make of object glass or eyepiece of the standard size Any make 

 can be used. There are absolutely no special requirements — a gJass or' 

 revolving nosepiece, an objective changer, or any form of apparatus eyepiece 

 can be employed. 



The interocular distance is varied by turning the milled head The 

 on the direct tube of the microscope (Fig. 108), this causes both ^jj^oe'"' 

 drawtubes to move in or out and alters the distance between the 

 oculars from 2 inches to 2^ inches, which, as the observer's eyes 

 cannot be in contact with the eyepieces, represents interocular 

 distances of about 2J inches to 2| inches. The tube length is 

 the standard 160 mm. at an intermediate position. For those 

 whose eyes are farther apart than this, tubes can be so con- 

 structed that they give extra separation. 



If the two eyes of an observer are dissimilar, the necessary lens 

 to render them equal can be supplied in a cap to fit over the 

 eyepiece. This is a better plan than the separate focussing 

 adjustment provided in a binocular telescope, because to efEect 

 an alteration in focus by means of the microscope eyepiece 

 requires such a large amount of motion. 



The advantages of binocular vision are not only that a Binocular 

 stereoscopic relief can be obtained: the rest to the eyes prevents '"^°'^- 

 fatigue and improves the quality of the vision ; not only is more 

 seen, but the perceptive faculties are much more constant. It 

 is frequently fovmd that after a quarter of an hour's examina- 

 tion with a monocular microscope, the perception of fine detail 

 goes and does not return till after a pause. This does not seem 

 to occur with binocular vision, or at least to only a slight degree. 



A further and somewhat more serious consequence of mon- 

 ocular vision is that the employed eye generally loses its visual 

 intensity of light. In order to concentrate the attention 

 upon the employed eye, a stronger light thaji is wise is often 

 used, and by degrees an illumination that appears white to the 

 unemployed eye is only grey to the other. Most microscopists 

 who do not force thenoselves to use the two eyes alternately will 

 find that the perception of light is less with the eye which has 

 been most used. 



Doubt has been at times expressed as to whether a microscope stereofoopio 

 looking at an object with a single object glass can under any ^^'"^■ 

 circumstances give a really stereoscopic relief. Those who have 

 worked with a binocular microscope do not retain such a doubt, and 

 the explanation of the phenomenon is quite satisfactory. Suppose 

 that (Fig. 106) represents the objective and that an object 

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

 from the left-hand of this blade which enters the object glass at all 



