128 TRANSACTIONS OF THE 



A microscope having only one body, and which can only be 

 used with one eye at a time, is called monocular ; a microscope 

 having two tubes and eye-pieces, allowing an object to be viewed 

 simultaneously with both eyes, is called binocular. The advan- 

 tages of a binocular microscope over a monocular one are the 

 following : — 



1. It is pleasanter and less trying to the eyes, permitting con- 



sequently a far more prolonged use. 



2. Much greater focal depth is gained. 



3. A stereoscopic effect is obtained, giving the observer a more 



correct idea of the shape of the object under observation; and 



4. By its use the mind is relieved of the constructive effort 



which it has to make in order to realize the shape of the 

 object or its details, when viewed only with one eye. 

 Altogether, I cannot sufficiently advocate the use of the binocular 

 microscope ; every one who values his eyesight, and has to make 

 prolonged mici'oscopical observations, should use a binocular. An 

 observer using a monocular microscope can form no better idea of 

 the object he views than a man using one eye alone can judge of 

 distance and form in a strange place. I am aware that many 

 scientific men, especially some old microscopists, still deprecate the 

 use of the binocular, but this arises either simply from prejudice, 

 or because they have only seen instruments with faulty binocular 

 arrangements, or because their eyes have different foci. 



The best binocular arrangement is undoubtedly that invented 

 by 3Ir. F. R. Wenham in 1860, and consists of a very small prism 

 of peculiar form (Fig. 12), which is placed in a small frame in the 

 nose-piece or the lower end of the body of the microscope. The 

 lower surface of this prism intercepts exactly half of the cone 

 of light coming from the object-glass, and reflects them up the 

 slantmg tube, whilst the other half of the rays passes through the 

 reflected bundle of rays straight in the eye-piece above the object- 

 glass. By the withdrawal of the prism the binocular microscope 

 is instantly converted into a monocular one. The effect of this 

 splendid arrangement is truly stereoscopic ; unfortunately it can 

 only be used with low powers, namely, up to the oile half-inch 

 object-glass inclusive, and only then when the latter has not more 

 than 40° aperture. 



For high powers one of these small Wenham prisms would require 

 to be fitted immediately behind the posterior combination of lenses of 



