26 CONSTRUCTION OF THE MICROSCOPE. 



or corrected at the second bending, in the opposite direction, by the 

 third lens. 



Every increase in the number of lenses is attended with one draw- 

 back, from the circumstance that a certain portion of light is lost by 

 reflection and absorption each time that the ray enters a new medium. 



The combination of three lenses approaches so very close to the 

 object under observation as, indeed, to prevent the use of more than 

 three ; and this constitutes a limit to the improvement of the simple 

 microscope, for it is called a simple microscope, although consisting 

 of three lenses. 



Before we proceed to describe the simple microscope and its appen- 

 dages, it will be well to explain such other points in reference to the 

 form and materials of lenses as are most likely to be interesting. 



A very useful form of lens was proposed by Dr. Wollaston, and 

 called by him the Periscopic lens. It consisted of two hemispherical 

 lenses cemented together by their plane faces, having a stop between 

 them to limit the aperture. A similar proposal was made by Mr. 

 Coddington, who, however, executed the project in a better manner, by 

 cutting a groove in a whole sphere, and filling the groove with opaque 

 matter. His lens, which is the well-known Coddington lens, is shown 

 at fig. 21 : it gives a large field of view, which is equally good in all direc- 

 tions, as it is evident that the pencils a b and b a pass through under pre- 



* *> 



fig. 21. fig. 22. 



cisely the same circumstances. Its spherical form has the further ad- 

 vantage of rendering the position in which it is held of comparatively 

 little consequence. It is therefore very convenient as a hand mag- 

 nifier ; but its definition is, of course, not so good as that of a well-made 



