C. S. Hastings — New Type of Telescope Objective. 267 



Art. XXVII. — On a New Type of Telescope Objective especially 

 adapted for Spectroscopic Use; by Charles S. Hastings. 



The ordinary achromatic doublet as invented by Dolland in 

 the last century, is, as is well known, very far from complying 

 with the condition implied in its name. For telescopes of 

 small aperture, or even for those of very considerable aperture, 

 if a ratio of focal length to aperture as great as that customary 

 with Dolland be employed, the defect in color correction 

 is neither very conspicuous nor very harmful in ordinary 

 use. But if the apertures are very large, as in our modern 

 astronomical instruments, or if the length be reduced relatively 

 to the diameter of the objective, this defect of secondary color 

 aberration becomes very conspicuous and reduces the optical 

 efficiency of the instrument very materially. The maximum 

 inconvenience of the defect, however, falls upon the spectro- 

 scopist, who finds that, although the optical efficiency of his 

 instrument is independent of the wave-length of light which 

 he happens to be observing, the instrumental adjustments must 

 undergo frequent changes for adaptation to different portions 

 of the spectrum. Another familiar and obvious consequence 

 of the secondary color defect is the impracticability of adapt- 

 ing the same instrument to purposes of both eye and photo- 

 graphic observation. 



It follows, therefore, that the solution of the problem, appar- 

 ently first seriously undertaken by Fraunhofer, namely, to 

 devise an absolutely color-free objective, is, and has long been, 

 of continuously increasing moment. Fraunhofer failed; but 

 unless I greatly misapprehend the meaning of his own record 

 of his work, the effort led directly to the discovery of the 

 Fraunhofer lines and to the beginning of spectroscopy. It is 

 true that nowhere, as far' as appears in his published writings, 

 does he state that this was his aim ; but in view of his very 

 extended experiments in varying the constitution of his glasses, 

 of his studies of the minute dispersion characteristics of various 

 substances, and of the extraordinary skill and conscientiousness 

 in perfecting an instrument which has possessed no other error 

 of importance since his unequaled contributions to the art of 

 telescope-making, few will question the validity of the 

 inference. 



Doubtless many investigators since Fraunhofer's time have 

 attacked the same problem, but, so far as I am aware, without 

 any recorded success until the writer showed, in a paper pub- 

 lished in this Journal, vol. xviii, p. 429, that there were certain 

 glasses, unfortunately not then procurable, which would yield 



