42 CONSTRUCTION OF THE MICROSCOPE. 



the three act 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 ra, 

 will vary with its distance from 

 thence. 



" When, therefore, the correction 

 of the whole is effected for an 

 opaque object, with a certain dis- 

 tance between the anterior and middle combination, if they 

 are then put in contact, the distance between the object 

 and object-glass will be increased ; consequently, the ante- 

 rior combination will act more powerfully, and the whole 

 will have an excess of positive aberration. Now the effect 

 of the aberration produced by a piece of flat and parallel 

 glass being of the negative character, it is obvious that the 

 above considerations suggest the means of correction, by 

 moving the lenses nearer together, till the positive aberra- 

 tion thereby produced balances the negative aberration 

 caused by the medium. 



" The preceding 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, sepa- 

 rated, into a beam consisting of the component colours of 

 the ray, and that each beam is widely different in form. 

 This difference, being magnified by the power of the 

 microscope, readily accounts for the chromatic thickening 

 of the outline just mentioned. Therefore, to obtain the 



