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



in a triple combination an objective entirely free from color. 

 Since then the extraordinary increase in number of materials 

 at the command of the optician, resulting from the labors of 

 Dr. Schott of Jena, led the writer to return to the problem 

 with the results which were published in a paper in vol. xxxvii 

 of the same Journal, in 1889. A general method of dealing 

 with all such problems was there developed and a number of 

 triads were indicated which would yield the most favorable 

 results. It is true that those given included a phosphate glass 

 which was believed by the makers to be permanent and has 

 since proved not to be so ; but it was distinctly stated that the 

 table exhibited a considerable number of other promising com- 

 binations, and the general method of recognizing them was 

 pointed out. 



It may properly be stated here that the latter paper also 

 contained a general discussion of interesting double combina- 

 tions, one of which promised to be of great value to spectro- 

 scopists ; but the inability of glass makers to supply large disks 

 of the materials in question proved an unforeseen difficulty. 

 Still, the writer employed this construction for a number of 

 years for his spectrometer and only displaced it recently by an 

 improved type of objective. Professor Keeler has also 

 employed the same construction, made by Mr. Brashear with 

 my aid, satisfactorily in spectroscope work. 



The experiments with triple combinations which followed 

 the paper last named met with an unforeseen difficulty. The 

 particular, triad which promised most in theory proved to have 

 in one of its members a perishable glass. This might possibly 

 have been used by covering the .objectionable material by a 

 more permanent glass cemented to it, but this course is not 

 without risk, and certain defects, to be noted later, are not so 

 readily eliminated if this method be chosen. The only practi- 

 cable course seemed to lie in replacing this material by one 

 beyond suspicion, and much time was spent in investigating 

 the possibilities of this meams. It was found, as appears from 

 the paper cited above, that there was no difficulty in selecting 

 triads which would meet the analytical condition, insuring 

 complete elimination of color and subject to practical limita- 

 tions as regards permanency ; but the necessarily greater curva- 

 tures of the lens surfaces introduced a new source of imper- 

 fection, namely, chromatic difference of spherical aberration. 

 Of course, this, like all other errors, is present to a greater or 

 less degree in all optical instruments which depend in any way 

 upon refraction for their action. In telescopes, however, this 

 error has never been sufficiently great to betray itself to the 

 users, although clearly indicated by theory. Gauss, indeed, a 

 long time ago showed how to reduce this particular error to a 



