160 PRACTICAL TREATISE ON THE MICROSCOPE. 



convex on both sides. A second difficulty now arises, 

 termed chromatic aberration ; whatever be the form of the 

 surface of the lens opposed to the light, the material itself 

 win act upon different portions of each ray with different 

 forces, and separate the white light into a variety of colours ; 

 this effect is represented by fig. 113; L L, &c., are parallel rays 



of light falling upon a plano- 

 convex lens ; two of these, 

 one from the margin, the 

 other from nearer the centre, 

 are shown as dispersed and 

 coloured, as In the spec- 

 trum formed by a prism ; 

 ^ig- 113. these coloured rays will 



cross each other at S S, the violet, being more refrangible than 

 the red, will be brought to a focus nearest to the lens, so 

 that, besides the spherical, there is the chromatic aberra- 

 tion. Whenever a ray passes from one medium into 

 another more or less refractive, the dispersion is certain to 

 take place. As every lens, according to its figure, is more or 

 less subject to these two kinds of aberrations, it becomes 

 necessary to ascertain how such sources of error may be 

 remedied; this, as will presently be shown, can be accom- 

 plished to some extent by the employment of two or more 

 lenses instead of one, so as to divide the refraction 

 among a larger number of surfaces; the defect may also, 

 in some measure, be counteracted by diminishing the 

 aperture of the lens, by placing a stop or diaphragm be- 

 hind it, to cut off the peripheral rays, but this, in all cases, 

 is attended with loss of light, and although the lens defines 

 better, its penetrating power is reduced in a like proportion. 

 For all lenses of very low power that are employed with 

 simple microscopes for dissecting, the spherical and chromatic 

 aberration need hardly be considered; it is only when the 

 higher powers are required to be used singly or either of them 

 as object-glasses for the compound instrument, that the two 

 kinds of aberration must of necessity be done away with, or, 

 in other words, the aperture must be increased without inters 



