164 PRACTICAL TREATISE ON THE MICROSCOPE. 



answering as an object to the single lens, L M. The mag- 

 nifying power of the compound microscope depends, first, on 

 the front focus of the lens, C D, that is the distance, C A or 

 D B ; secondly, on its posterior focus, C B' or DA'; and 

 thirdly, on the focus of the lens, L M. The power may, 

 therefore, be increased by substituting a lens of shorter focal 

 length for C D or L M, or by making the distance between 

 C D and L M greater. By the addition of a second lens at 

 the eye end, the field of view is much increased, and we have 

 what is termed an eye-piece; but by this arrangement no- 

 thing has been done to diminish either the spherical or the 

 chromatic aberration of the object-glass, C D. One very 

 great advantage which the compound has over the simple 

 microscope, besides the greater magnifying power, is that the 

 field of view is large and almost equally good in all directions, 

 whereas, in the simple, the field is small, and only good in the 

 centre. But however well an instrument of this kind may be 

 constructed, it will fail to exhibit even many of the ordinary test 

 objects; it is, therefore, unfit for the purposes of scientific 

 investigation, for in the diagram given at fig. 115, it must not 

 be supposed that the image of the object formed at B' A is a 

 correct one ; on the contrary, a space for some distance, both 

 above and below it, may be considered as occupied by an 

 infinite number of variously coloured images of different sizes, 

 that interfere with each other, and necessarily render the 

 definition indistinct. To overcome all these difficulties has 

 been a labour of years, and for an account of the various steps 

 towards improvement which have taken place, the reader 

 must consult the latter part of the History of the Microscope, 

 beginning at page 32, where these points are described in 

 chronological order. 



The first attempts to achromatise the object-glass were not 

 very successful ; and we find that within the last twenty-five 

 years such philosophers as Biot and Wollaston predicted that 

 the compound microscope would never excel the simple when 

 supplied with doublets. Happily for us such opinions have 

 been since found to be groundless, "and the compound micro- 

 scope," says Mr. Ross, ," within the last fifteen years, has 



