Scientific Lectures. 41 



converging pencils before they reach their focus sends them to another 

 and nearer focus where the curvature is reversed. The two foci are 

 in fact conjugate, but one is negative to the other. Tlie image thus 

 formed by the fiekl-glass is concave toward the eye ; and this is the 

 kind of image to which tlie conjugate seen tlirougli the eye-giass is 

 flat, or less rounded, or rounded the other way. It is a mathematical 

 problem easily solved to assign the curvatures to the several glasses, 

 which shall produce exactly the flatness of field desired. But we are 

 not yet done with all the useful eflects of this admiral)le combina- 

 tion. The eye-glasses may produce color as well as the object glasses. 

 To make these achromatic also would require unnecessary trouble 

 and expense, since the same efiect may be produced in another mode, 

 which I will now describe. The object glasses are . corrected for 

 color in excess. • That is to say, the blue image of the corrected glass 

 is formed further from the lens than the red image. The reverse is 

 the case with the uncorrected glass. The diagram before us shows 

 the relative positions of the two images. These two images are 

 somewhat approximated by the convergency produced by the iield- 

 glass, which, being uncorrected, acts more powerfully on tlie blue 

 than on the red. The mai'gins of the two images are also brought 

 into straight line wath the axes passing througli the center of the eye- 

 glass, which axes are the boundaries of the negative resultant image 

 finally seen by the observer. The eye-glass, acting like the field- 

 glass, more powerfully upon the blue than upon the red, brings both 

 at last into perfect coincidence, so that the ultimate image is divested 

 of all its imperfections : it is without aberration, without distortion, 

 without color, and without curvature. 



These results are not by any means secured with the facility watli 

 wdiich I speak of them here. The utmost dehcacy is necessary in 

 the adjustment of the several parts ; the most careful computations 

 are indispensable for the determination of each curvature ; and, 

 finally, the most exquisite artistic skill must be laid under contri- 

 bution to realize in e>^ecution the exactions of theory. If to any 

 production. of the combined resources of art and science the term 

 masterly is justly applicable, it is so in an eminent degree. to the 

 modern perfected microscope. The question is sometime asked, what 

 are the tests by which a good microscope may be recognized I It 

 may be answered that a microscope should be chosen with reference 

 to the purpose to which it is to be applied; but that there is one 

 quality without which no microscope is valuable for any purpose. 



