64 THE MICROSCOPE. 



quently the rays from the object arc diverging from a 

 point at a greater distance than when the combina- 

 tions 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 correc- 

 tion 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 com- 

 bination w r ill act more power- 

 fully, and the whole will have 

 an excess of positive aberration. 

 Now the effect of the aberra- 

 tion produced by a piece of flat 

 and parallel glass being of a negative character, it 

 is obvious that the above considerations suggest the 

 means of correction, by moving the lenses nearer to- 

 gether, and the positive aberration is made to balance 

 the negative aberration caused by the medium. 



The preceding refers only to "the spherical aberra- 

 tion ; but the effect of the chromatic is also seen when 

 an object is covered with a piece of glass : it pro- 

 duces chromatic thickening of the outline of 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, separated, into a beam con- 

 sisting of the component colours of the ray, and that 

 each beam is widely different in form. This difference, 

 being magnified by the objective of the microscope, 

 readily accounts for the chromatic thickening of the 

 outline just mentioned. Therefore, to obtain the finest 

 definition of extremely delicate and minute objects, 



