240 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



general construction easily understood. For high magnifying powers 

 such an instrument would be altogether worthless for want of good 

 definition. It is possible, on this .system, as in the case of the simple 

 lens, to partially correct the errors either of the object-glass or of 

 the eye-piece with regard to spherical and chromatic aberration, 

 but far more complex arrangements are necessary to obtain first-rate 

 results with high powers. 



The first fault is corrected by limiting the extent of the real image 



by means of a diaphragm placed at the 

 focus of the eye-piece that is, at ah. 

 But as this also limits the field of the 

 microscope, an eye-piece of large dia- 

 meter is used, having in consequence 

 a more extended field. To the same 

 end an eye-piece is used with a system 

 of two plano-convex lenses, one called 

 the field lens and the other the eye lens, 

 the convexity of which is away from the 

 eye. This is Campani's eye-piece, Fig. 

 178, in which the chromatic aberration 

 is somewhat diminished. SI is a lumi- 

 nous ray proceeding from the object ; 

 on being refracted, it is divided into 

 coloured rays, the red following the 

 direction IE, and the violet IV, so that 

 the eye would see the edge of the 

 FIG. 177,-path of the luminous rays in object coloured if the second eye-piece 



the compound microscope. , . , , , 



did not make the coloured more 

 parallel at B', where the eye is placed to make the observations. 



Achromatism is also obtained by making the object-glass of two 

 lenses, one of flint and the other of crown glass, the latter bi-convex, 

 and the former divergent (Fig. 179), the curves being so regulated 

 that the greater dispersive power of the flint glass to a great extent 

 counteracts the less dispersive power of the crown glass, but only 

 partially counteracts the magnifying power. 



The best modern object-glasses are, however, far more complex. 

 Until quite recently they were constructed of three sets of lenses, 

 each an approximately achromatic combination of a plano-concave of 



