174 PRACTICAL TREATISE ON THE MICROSCOPE. 



which, as before stated, interferes with the previously balanced 

 aberrations of the several lenses composing the object-glass. 

 There are many cases of this, but the one here selected serves 

 best to illustrate the principle. If an object-glass is con- 

 structed as represented by fig. 120, where the posterior com- 

 bination, P, and the middle, M, have together an excess of 

 negative aberration, and if this be corrected by the anterior 

 combination, A, having an excess of positive aberration, then 

 this latter combination can be made to act more or less power- 

 fully upon P and M, by making it approach to or recede 

 from them; for when the three are in close contact, the 

 distance of the object from the object-glass is greatest, and, 

 consequently, the rays from the object are diverging from a 

 point at a greater distance than when the combinations are 

 separated, and as a lens bends the rays more, or acts with 

 greater effect, the more distant the object is, from which the 

 rays diverge, the effect of the anterior combination A, upon 

 the other two, P and M, will vary with its distance from 

 thence. When, therefore, the correction of the whole is 

 effected for an opaque object, with a certain distance between 

 the anterior and middle combination, if they are then put in 

 contact, the distance between the object and the object-glass 

 will be increased ; consequently the anterior combination will 

 act more powerfully, and the whole will have an excess of 

 positive aberration. Now, the effect of the aberration pro- 

 duced by a piece of flat and parallel glass being of the nega- 

 tive character, it is obvious that the above considerations 

 suggest the means of correction by moving the lenses nearer 

 together, till the positive aberration thereby produced balances 

 the negative aberration caused by the medium. The pre- 

 ceding refers only to the spherical aberration, but the effect 

 of the chromatic is also seen when an object is covered with a 

 piece of glass ; for in the course of my experiments, I observed 

 that it produced a chromatic thickening of the outline of the 

 Podura and other delicate scales, and if diverging rays near 

 the axis and at the margin are projected through a piece of 

 flat parallel glass, with the various indices of refraction for 

 the different colours, it will be seen that each ray will emerge 



