REPORT OF COMMITTEE ON OBSERVATORIES 8 1 



It is well known that the maximum efficiency of a very large tel- 

 escope can be secured only under excellent atmospheric conditions. 

 It is also true that a large aperture is required for the most critical 

 differentiation of the various qualities of seeing. It may very well 

 happen that what may appear to be very good seeing with a 6-inch 

 telescope will not prove so with an instrument of the largest dimen- 

 sions. Moreover, different kinds of work vary in their require- 

 ments. For example, with a comparatively short photographic 

 telescope no perceptible difference in the results will be found on 

 nights of excellent definition and on those when the seeing is only 

 fairly good. Again, for meridian-circle work the most essential 

 condition is a steady image, and for difficult double-star work there 

 is the additional requirement of fine definition. 



Even though some classes of work may be carried on very success- 

 fully under circumstances which are not altogether favorable, it is 

 nevertheless true that they would be more easy of accomplishment 

 under excellent conditions ; and it is also true that many important 

 investigations require the highest obtainable efficiency of the most 

 powerful telescopes for their successful prosecution. That site, 

 therefore, which affords excellent seeing the most continuously, 

 and the necessities, conveniences, and comforts of life the most 

 abundantly, will be the best adapted for the needs of a great 

 observatory. 



For several years I have used the 36-inch and 12 -inch telescopes 

 of the lyick Observatory for double-star and other observational 

 work, and on many occasions when working with one instru- 

 ment I have gone to the other in order to make a comparison of 

 the two. Whenever the seeing appears excellent with the large 

 telescope it also appears excellent with the smaller one ; but when 

 it appears only fairly good with the 36-inch there is a tendency to 

 rate it somewhat higher with the 12-inch. As soon as the tests 

 began with the 9-inch it was further noted that there is a very 

 perceptible difference between it and the 12-inch in the same direc- 

 tion. It was my especial object while the instrument was set up at 

 Mount Hamilton to become acquainted with the characteristics of 

 the image formed with the 9-inch under varying conditions, in order 

 that I might properly interpret them and retain as far as possible 

 36-inch standards of excellence while making the tests in the va- 

 rious localities. To have proceeded in the opposite direction, by 

 capping down the larger instruments to a 9-inch aperture would, in 

 my opinion, have resulted in a lowering of the standard which it is 



