THE MICROSCOPE. 33 



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

 serves best to illustrate the principle. I need not encumber 

 the description with the theoretical determination of this quan- 

 tity, as it varies with exceedingly minute circumstances which 

 we cannot accurately control; such as the distance of the object 

 from the under side of the glass, and the slightest difference in 

 the thickness of the glass itself; and if these data could be 

 readily obtained, the knowledge would be of no utility in 

 making the correction, that being wholly of a practical nature. 



"If an object-glass is constructed as represented in Fig. 16, 

 where the posterior combination P and the middle M have 

 together an excess of negative aberration, and if this be cor- 

 rected by the anterior combination A, having an excess of posi- 

 tive aberration, then this latter combination can be made to act 

 more or less powerfully upon P and M, by making it approach 

 to or recede from them; for when the three are in close con- 

 tact, 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 dis- 

 tance between the object and object-glass will be increased; 

 consequently the anterior combination will act more power- 

 fully, 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 aberration 

 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 cov- 

 ered with a piece of glass; for, in the course of my experiments, 

 I observed that it produced a chromatic thickening of the out- 



