40 Transactions of the American Institute. 



object-glass tlie lenses are so placed, that when the object is in the 

 short aplanatic focus of the first glass, the negative image is in the 

 longer aplanatic focus of the second and third, considered as one, 

 and the aberration is suppressed. But a more important fact is, that 

 the oblique rays, for wliich the aberration as already stated is not 

 corrected with any single lens, is such as to produce two opposite and 

 counteracting effects in the two points just mentioned ; these oblique 

 pencils of rays tln*owing a luminous coma outward at the longer 

 focus, and a similar coma inward, at the shorter. The two effects 

 balance, and thus the compound objective produces an image free 

 from aberration throughout. Another advantage is secured by the 

 compound objective. "When the object is viewed through a glass 

 cover, even though the cover be not more than the one-hundredth 

 or one-two hundredth of an inch thick, tlie pencils coming from it 

 are made (as has earlier been stated) aberrant. The aberration pro- 

 duced is negative. If the front lens be slightly withdraMm, the 

 others remaining stationary, the object will fall into that space 

 between the two aplanatic foci ; in which the aberration produced is 

 positive. The two effects counterbalance each other, and the perfec- 

 tion of the image is once more restored. The corrections here 

 described are founded on discoveries made by Mr. J. J. Lister, of 

 London, and communicated to the Eoyal Society in 1830. They gave to 

 the instrument a power and perfection previously unknown, and con- 

 stitute the essential feature of all modern first-class objectives. But 

 though aberrations are thus corrected, the curvature of the image 

 remains to be removed. This is accomplished by invoking tlie aid 

 of the eye glass, or, as it is more properly called, the eye-piece. The 

 eye-piece is compound as well as the objective. It consists essenti- 

 ally of two plano-convex lenses ; the smaller, nearest the eye, being 

 the eye-glass proper, and the other being called, from the use which 

 it was originally designed to subserve, the field-glass. It bends the 

 pencils proceeding from the object glass inward, as is seen on the 

 diagram, and gives the eye a better command of them. It thus 

 enlarges the field of view, though reducing somewhat at the same 

 time the size of the image. But it accomplishes a much more 

 important object than this. The image formed by the rays from the 

 object glass is seen to be curved, presenting its convexity towai'd the 

 eye. This is the most unfavorable J)osition in which to view it, since 

 the eye-glass itself tends to produce a convexity in even a flat object 

 in the same direction. But the field-glass receiving the rays of the 



