C. 8. Hastings — New Type of Telescope Objective. 269 



residual of a higher order of minuteness, but the fact that his 

 construction has never come into use is a most convincing proof 

 that the error is quite negligible as compared to other defects 

 inherent in the ordinary construction. But when we try to 

 make a color-free triple objective after the methods of the 

 paper of 1889, we find that the defect in question becomes of 

 great moment. Especially is this true if we prescribe the 

 cementing of the objective so that there shall be only two free 

 surfaces. Such an objective, if corrected as regards spherical 

 aberration for light-waves of mean length, would have strong 

 positive spherical aberration for the red and negative for the 

 violet ends of the spectrum. It is true that this defect might 

 not prove very obvious for a telescope which is to be used only 

 for objects which are approximately white, but it would be 

 intolerable in spectroscopic use. The obvious method of 

 reducing the error is to increase the ratio of the focal length to 

 the aperture. This method, however, would introduce such 

 serious structural and mechanical difficulties, and so far reduce 

 the convenience of handling all spectroscopes to which it might 

 be applied, that it seemed to me quite impracticable. 



As a possible means of securing the end in view, convinced 

 as I am that its importance warrants any amount of labor, I 

 lately turned to a consideration of the possibilities possessed by 

 a combination of four varieties of glass. The investigation 

 is necessarily somewhat laborious as appears from the unusual 

 conditions imposed from the outset ; but the time expended in 

 attaining complete success was short compared to the protracted 

 investigations which led to a definitive rejection of the triplet 

 as quite inadequate. In short, I have constructed an objective, 

 consisting of a quadruple combination of silicate flint, borosili- 

 cate flint, silicate crown and barium crown, which possesses all 

 the properties demanded. It has but two free surfaces, the 

 four lenses being cemented together. With an aperture of 

 one-tenth the focal length, its focal plane is rigidly the same 

 for all wave-lengths, from that of the Fraunhofer line A to 

 that of K, while it is sensibly free from chromatic differences 

 of magnification and of spherical aberration. With its perfect 

 color correction, the well-known (but ordinarily overlooked) 

 chromatic aberration of the eye becomes very sensible. This, 

 however, I have eliminated by means of a specially devised 

 ocular, so that in my instrument there is no reason why its 

 length may not be made permanently invariable. One notable 

 advantage in the construction will appear at once to all spec- 

 troscopists : wave-surfaces from the collimator being rigidly 

 plane for all wave-lengths, the adjustment of the prisms for 

 minimum deviation — provided always that their faces are 

 accurately plane — ceases to be of importance. Thus a con- 



