50 THEORY OF THE MICROSCOPE. 



one or both of the divided pencils with other prisms of similar 

 construction. 



The multiplication of the objective-image is, of course, accom- 

 panied by a corresponding diminution of light ; and a further loss 

 of light occurs by absorption in the media employed. On this 

 account only low-power objectives are applicable to multocular 

 Microscopes, and even these, in the opinion of competent judges, 

 leave much to be desired. 



[Two short paragraphs in the original are here omitted, 

 relating to Stereoscopic Binocular Microscopes, in which brief 

 mention is made of three forms Nachet's, Wenham's, and 

 Hartnack's (illustrated by Fig. 17). ED.] 



V. 



CHROMATIC AND SPHERICAL ABERRATION. 



1. CHROMATIC ABERRATION. 



IT, is well known that the different coloured rays of which white 

 light is composed are unequally refracted in their passage through 

 a refracting medium. In a given system of lenses there is, 

 therefore, for each separate colour a different position of the 

 cardinal points, and, consequently, also of the final image. The 

 confusion which results from this circumstance is known as 

 chromatic aberration. Its elimination is rendered possible, as we 

 assume to be known, by the property of refracting substances of 

 so acting upon the different colours as to disperse them in a very 

 unequal degree, though the difference in the refractive power is 

 very slight. With crown-glass of mean refractive index 1*5342, 

 and with flint-glass of mean refractive index 1*649, we get, for 

 instance, as the index of refraction for the extreme rays, 



Crown-glass. Flint-glass. 



Red 1-5258 1-6277 



Violet 1-5466 1-6711. 



The difference between these extremes is with crown-glass -0208, 

 and with flint-glass '0434. These two kinds of glass differ, there- 



