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



struction, which must have occurred to every one who has 

 seriously studied the theory of the spectroscope, in which 

 the last prism of a train is of half the angle of the remainder 

 and silvered on the back, so that the light retraces its course 

 through the train, becomes entirely practical. Indeed, my 

 experiments with the new telescope lead me to prefer a con- 

 struction of spectroscope in which the collimator and telescope 

 are set at constant angle, and the prisms, arranged as above, 

 are alone movable. This is the familiar construction of the 

 grating spectroscope. 



Although the objective described above consists of four 

 lenses, I imagine that a cemented system of five lenses would 

 in some cases be preferable, especially in relatively large aper- 

 tures ; but there is no doubt in my mind that four kinds of 

 glass are sufficient and, unless greater structural complexity is 

 admitted, necessary for the ends defined. 



Should the construction meet my confident expectation and 

 supply the spectroscopist with an optical instrument combining 

 the merits of a reflector with the greater merits of a refractor, 

 it will be convenient to give it a characteristic name suggested 

 by its properties. These are, as given above, chromatic differ- 

 ences of focal distance, of focal length, and of spherical aberra- 

 tion, all reduced to practically zero, together with a minimum 

 possible number of free surfaces. As such an objective is the 

 same in its action upon light of all wave-lengths, I propose to 

 call it an isokumaiic system. 



Mr. Brashear of Allegheny, who made for me the prisms for 

 the study of these glasses, as well as scores of others, and who 

 has shown unfailing good nature and constant readiness to 

 lend me his efficient aid in all my optical investigations, merits 

 my unstinted acknowledgments. I have promised the neces- 

 sary calculations if he is called upon to carry out for others 

 this difficult piece of optical work which has yielded so 

 much satisfaction to the writer. 



Yale University, March, 1899. 



