174 CARNEGIE INSTITUTION OF WASHINGTON. 



Spectroscopic Observations. — The spectroscope used with the 

 coelostat telescope is of the Littrow form— a single lens, of 4 inches 

 aperture and 18 feet focal length, serves at once as collimator and 

 camera lens. After passing through the slit, which is mounted in the 

 focal plane of the photographic objective employed with the coelostat, 

 the rays pass to the 4-inch objective, by which they are rendered 

 parallel. They then meet the 4-inch Rowland plane grating, having 

 14,438 lines to the inch, from which they are returned through the 

 4-inah objective. The image of the spectrum is formed on a photo- 

 graphic plate, mounted in the focal plane and a little to one side of 

 the slit. This apparatus is giving excellent definition, surpassing 

 that of any spectroscope employed at the Yerkes Observatory. 



The character of the results obtained with this spectroscope, and 

 its convenience of manipulation, illustrate one of the arguments in 

 favor of fixed telescopes of the coelostat type, as contrasted with 

 moving equatorial telescopes. At the Yerkes Observatory it has 

 never been possible to attach a sufficiently long and powerful spec- 

 troscope to the moving tube of the 40-inch refractor. Such a 

 spectroscope must be mounted in a fixed position on substantial 

 piers, and the telescope must be so constructed as to permit a sharp 

 and well-defined image of the sun to be maintained in a fixed position 

 on the slit. This can readily be accomplished with the aid of a coelo- 

 stat, provided only that the difficulties peculiar to this type of tele- 

 scope can be overcome. From the experiments so far made, we believe 

 that the difficulties can be surmounted and that the fixed telescope is 

 certain to become an instrument of great importance in the future. 



CONCLUSION. 



From the observations given in this paper, it appears that Mount 

 Wilson meets in a very remarkable degree the requirements of a 

 site for a solar observatory. Indeed, I know of no other site that 

 compares at all favorably with it. If a large solar observatory were 

 established there, it might be expected to yield many important re- 

 sults, not to be obtained under less favorable conditions. 



