Tlie Achromatic Telescope. 451 



of the continental air may bring up the performance of its 

 opponent considerably higher than would be expressed by the 

 ordinary proportion of the squares of the linear apertures. 



THE ACHROMATIC TELESCOPE. 



In our last paper upon the subject, we brought down our 

 notice of this beautiful invention to the completion of its 

 theory, when through the intellectual and manual skill of the 

 elder Dollond, both aberrations, chromatic and spherical, were 

 destroyed by the combination of a concave lens of flint with a 

 convex one of crown (now of plate) glass. In this form they 

 were made for some time with a moderate degree of success. 

 A difficulty, however, in the correction of the spherical aberra- 

 tion, which, though much the smaller in amount, is apt to be 

 the more troublesome, induced the inventor subsequently to 

 divide the convex lens in two, leaving one half in front of the 

 flint lens, and placing the other at the back of it. The reason 

 of this procedure may be easily explained. If a convex lens 

 having a given focal length is replaced by two placed together, 

 whose combined focal length equals it, each of them taken 

 singly will have a focus of twice the length, and the curves 

 belonging to these longer foci will, of course, be much 

 shallower than those of the original single lens. But the 

 spherical aberration diminishes so rapidly with the decreasing 

 depth of curvature that, under favourable circumstances, a 

 division of a convex lens into two producing the same com- 

 bined focus will reduce the aberration to a quarter of its previous 

 amount ; and though this might not be the case in an object- 

 glass where the best form or position for the divided lenses 

 might be inconvenient, yet the reduction would be sufficient 

 to accomplish perfectly the intended object — the bringing of 

 the convex completely under the control of the concave 

 aberration. The additional loss of light from reflection at two 

 more surfaces, and from transmission through a greater thick- 

 ness of glass — and the crown glass of those days was very 

 green — as well as the additional risk of defective workmanship 

 or centring, did not prevent this triple arrangement from being 

 considered the best possible, and many instruments were con- 

 structed of a nominally 3§ inch aperture, that is, of this 

 measure previous to setting, but really reduced by the cell to 

 about 3 T 7 inches, to which a focus of 3 § or 5 feet was given. 

 These bore a very high character : it may be questioned, how- 

 ever, whether their reputation was not in some measure due to 

 the contrast with their predecessors; and there is no doubt 

 that they are surpassed at the present day. More recently 

 opticians, both English and foreign, have reverted to the 

 original double construction, and, as is evident, without any 



VOL. VI. — NO. VI. G G 



