MOUNT WILSON OBSERVATORY. 261 



3 inches focal length. The latter instrument can be mounted on the 

 double-slide plate-holder, which can thus be used for guiding during 

 exposures. 



In order to determine precisely what advantages may be expected 

 from the use of the 100-inch telescope, it has been submitted to a 

 comparative test with the 60-inch telescope, the qualities of which 

 are known from long experience. When feasible, the observations 

 required for such tests are made simultaneously with the two telescopes, 

 which are not far apart on Mount Wilson, and are therefore subject to 

 the same atmospheric conditions. In the case of an instrument of the 

 great aperture and equivalent focal length of the Hooker telescope, 

 when used at the 134-foot focus, the chief outstanding question, after 

 the rigorous optical and mechanical requirements have been met, is 

 the quality of the atmosphere. In stellar spectroscopy the problem 

 is whether the star images, at the 134-foot focus, will be so small as to 

 permit essentially all of the light to pass through the slit during the 

 exposure. If this can be accomplished, the Hooker telescope should 

 give about 2.8 times as much light as the 60-inch telescope, thus render- 

 ing possible the observation of stars about one magnitude fainter than 

 those within the reach of the latter instrument. 



To answer this question, a comparative simultaneous test was made 

 on the evening of August 13, 1919. The spectrum of the star e 

 Andromedse was photographed on Seed 30 plates from the same box 

 with the two telescopes, using spectrographs having nearly identical 

 optical constants, mounted at the Cassegrain focus. ^ For the 60-inch 

 the equivalent focal length at this point is 80 feet, while for the 100- 

 inch, as already remarked, it is 134 feet. Both spectrographs have 

 collimators of 2.5 inches aperture and 40 inches focal length, and 

 cameras of 4 inches aperture and 18 inches focal length. In each case 

 a single 63° prism of 0.102 Jena glass is used, giving a dispersion of 36a 

 to the millimeter with the camera employed. The slit-widths of the 

 two spectrographs were varied until the photographic resolution, de- 

 termined by comparing close pairs of lines in the spectra of the same 

 star, was found to be the same. The exposures on e Andromedse were 

 made by Mr. Stromberg with the 60-inch and by Mr. Merrill with 

 the 100-inch, under good conditions of seeing, and the plates were 

 afterwards developed together. 



Thirty-three spectrograms were secured for comparison. From 20 

 of these Mr. Stromberg finds the following mean ratios of exposure- 

 times required to give the same intensities and photographic resolution 

 with the two instruments: 



Region X4000 X4300 X4500 



Mean ratio 4.5 3.2 3.3 



^ The spectrograph of the 60-inch telescope contains a single plane mirror, not duplicated in the 

 spectrograph of the 100-inch telescope. 



