MAGNirTING POWERS. 



163 



where any error, however slight, in the object-glass, is further 

 magnified by the eye-piece. The construction of a magni- 

 fying power for the compound microscope is, therefore, a 

 more complicated matter than that 

 either of a doublet or triplet ; for not 

 only must several lenses be employed 

 to form one magnifier, but even two 

 different kinds of glass must be used 

 for one lens of such magnifier; and 

 throughout the whole range of optical 

 science, perhaps there is no single pro- 

 blem that has ever yet engaged so 

 much of the attention of the learned, 

 in all countries, as that of achromatism ; 

 and of all the triumphs in science that 

 have been achieved by a combination 

 of the labours of the mathematician and 

 the workman, no one can outvie in 

 delicacy of construction and in impor- 

 tance a well made achromatic combi- 

 nation. 



In order to understand the different 

 parts entering into the composition of a 

 perfect object-glass for the compound 

 microscope, it must be first shown how 

 the rays of light comport themselves in 

 their passage through a compound mi- 

 croscope of the ordinary kind, that is 

 not achromatic. Fig. 115 represents a 

 compound microscope of two lenses, 

 A B being an object, C D the object- 

 glass, and L M the eye-glass. The 

 rays proceeding from the object, A B, 

 are acted on by the lens, C D, and are 

 brought to a focus at B' A' ; but not 

 being intercepted there, they cross and pass on till they reach 

 the eye-glass, L M, by which they are rendered nearly 

 parallel, and as such reach the eye, the image, B' A', 

 11* 



