CONSTRUCTION OF THE MICROSCOPE. 41 



will be the greatest distance between the object and the 

 object-glass. The focal length is -| of an inch, having an 

 angular aperture of 60 , with a distance of -^ of an inch, 

 and a magnifying power of 970 times linear, with perfect 

 definition on the most difficult Podura scales. I havs 

 made object-glasses ^ of an inch focal length ; but as the 

 angular aperture cannot be advantageously increased if 

 the greatest distance between the object and object-glass is 

 preserved, their use will be very limited. 



" The quality of the definition produced by an achro- 

 matic compound microscope will depend upon the accuracy 

 with which the aberrations, both chromatic and spherical, 

 are balanced, together with the general perfection of the 

 workmanship. Now in Wollaston's doublets and Holland's 

 triplets there are no means of producing a balance of the 

 aberrations, as they are composed of convex lenses only ; 

 therefore the best thing that can be done is to make the 

 aberrations a minimum. The remaining positive aberra- 

 tion in these forms produces its peculiar effect upon 

 objects (particularly the detail of the thin transparent 

 class), which may lead to misapprehension of their true 

 structure ; but with the achromatic object-glass, where 

 the aberrations are correctly balanced, the most minute 

 parts of an object are accurately displayed, so that a satis- 

 factory 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 conse- 

 quently deteriorate the definition; and this defect will be 

 more obvious from the increase of distance between the 

 object and object-glass. 



" If an object-glass is constructed as represented in 

 fig. 27, where the posterior combination p and the middle 

 m have together an excess of negative aberration, and if 

 this be corrected by the anterior combination a having an 

 excess of positive 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 



