32 



CONSTRUCTION OF THE MICROSCOPE. 



satisfactory judgment of their character may be formed. When an 

 object has its aberrations balanced for viewing an opaque object, and it 

 is required to examine that object by transmitted light, the correction 

 will remain; but if it is necessary to immerse the object in a fluid, or 

 to cover it with glass, an aberration arises from these circumstances 

 which will disturb the previous correction, and consequently deteriorate 

 the definition ; and this defect will be more obvious with the increase 

 of distance between the object and object-glass. 



"If an object-glass is constructed as represented in fig. 26, where 

 the posterior combination p and the middle m 

 have together an excess of negative aberration, 

 and if this be corrected by the anterior combi- 

 nation a having an excess of positive aberra- 

 tion, 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 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 di- 

 verge, 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 object-glass will be increased ; consequently the anterior 

 combination will act more powerfully, and the whole will have an ex- 

 cess 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 correc- 

 tion, by moving the lenses nearer together, till the positive aberration 

 thereby produced balances the negative aberration caused by the me- 

 dium. 



" 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 j and if diverging rays near the axis and at the margin are pro- 



