192 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1923 



which are constantly arising in various fields. Such beautiful new 

 instruments as the X-ray spectrograph or the mass spectrograph of 

 Aston, while perhaps not directly applicable in astronomy, may con- 

 tain hints, and also yield results, which can be used to advantage. 



The above considerations will help to explain the somewhat un- 

 orthodox equipment and policy of the Mount Wilson Observatory. 

 We have tried from the outset, with the valuable cooperation of our 

 research associates, to at least utilize the more obvious possibilities 

 offered by the progress of physics and chemistry, and to gain such 

 advantages as laboratory conditions and methods placed at our dis- 

 posal. Hence the design of the Snow and tower telescopes, equipped 

 for solar research; the coude principle and constant temperature 

 laboratories of the 60-inch and 100-inch reflectors,* arranged for the 

 photography of stellar spectra under high dispersion, and for in- 

 vestigations like those with the thermopile, bolometer, and radio- 

 meter on stellar radiation and ejiergy spectra; the exceptional care 

 taken to secure smooth rotation of the 100-inch dome in order to 

 diminish the vibration of the high dispersion stellar spectograph 

 (soon to be mounted on its pier) during exposures continued for 

 several nights; the construction of the ruling machine, one of the 

 prime purposes of which is to permit such experiments as have just 

 rendered possible the concentration of most of the incident light in 

 any desired order of spectrum ; the development of the stellar inter- 

 ferometer, first in conjunction with the 100-inch telescope and now 

 as a separate instrument. Hence the provision of machine and 

 optical shops adequate for a wide range of constructional work and 

 a physical laboratory in which to conduct researches required for the 

 interpretation of celestial phenomena. Hence also our close coopera- 

 tion with the California Institute of Technology, the recent growth 

 of which as a research institution is so advantageous to the observa- 

 tory. 



Looking ahead, and speculating on the possibilities of future in- 

 struments, it may be mentioned that comparative tests of the 60-inch 

 and 100-inch telescopes promise well for larger apertures. Their 

 practicability, so far as this depends upon atmospheric limitations, 

 can be fairly well tested by observations of the united star images 

 given by the two mirrors of a stellar interferometer at increasing 

 separations. The production of large mirror disks is another prob- 

 lem. Fused quartz mirrors, if they can be made of sufficient dimen- 

 sions, will be extremely valuable for solar telescopes and large re- 

 flectors because of their low coefficient of expansion, but for moderate 

 apertures pyrex glass has already proved a fairly effective substi- 

 tute. As for the stellar interferometer, I believe it will ultimately 

 attain apertures of 100 feet or more, possibly in some fixed form, 



