170 PROCEEDINGS OF THE AMERICAN ACADEMY 



judges, they have exhibited a more complete achromatism, and in 

 otlier respects more perfect definition, than was to be found with object- 

 glasses of the ordinary form, of equal dimensions. 



Some part of this superiority may be attributable to the manner of 

 mounting the lenses, which admits of readily changing their relative 

 positions so as to effect the best adjustment by actual trial ; a provision 

 undoubtedly of considerable value, but perhaps equally applicable in 

 the old system, if a slight separation of the inner surfaces of the 

 crown and Hint lenses were made one of the conditions for determininor 

 the curves. By this means, as Steinheil has remarked, we may not 

 only diminish outstanding errors in the object-glass, but also, to some 

 extent, the aberrations of the eyepiece, and even defects in the eye 

 itself. Thei'e seems, however, to be no reason to doubt that these 

 object-glasses owe their excellence mainly to the improved theory of 

 their curves. 



Among other advantages, the new combination admits of larger 

 angles of aperture than would otherwise be practicable, without com- 

 promising the clearness of the definition. It is here, in fact, that the 

 value of the improvement is best illustrated. Any shortening of the 

 focal length accomplished without sacrificing illuminating power, or 

 defining qualities, is a substantial gain in more than one direction. 

 It reduces the telescope to a more manageable size, which, in one of 

 the larger class, is a matter of the first importance, for not only is the 

 size of the dome and building required to protect it, and, in general 

 the cost of all the accessory apparatus necessary for its efficiency, 

 largely diminished by a reduction in the length of the focus, but the 

 facility of using it depends also very much on the same condition. 

 Again, by shortening the tube, we apply the only really effectual 

 means of reducing the flexure, — the most intractable of all sources 

 of error in meridian instruments. 



Hitherto, in the practice of the best opticians, the apertures of the 

 largest object-glasses have not exceeded -j-'^- of the focal length, which 

 is the proportion in Mr. Clark's 18.5-inch lens. With those of mod- 

 erate size the ratio of y'^ to yj has been successfully employed. Of 

 such, the object-glasses of the vertical circle and prime vertical instru- 

 ment at Poulkova, of 6-inch aperture, are examples of remarkable 

 excellence. At present, however, Messrs. IMerz are prepared to ex- 

 tend the ratio of J^ even to lenses of 19^ inches (English) aperture; 

 a gain in the surface exposed to the light for the same focal length, of 



