56 CARNEGIE INSTITUTION 



the sun will remain fixed in the field of view for hours together. 

 Attachments weighing as much as 700 pounds may be carried at the 

 lower end of the tube, but it is out of the question, in spite of the 

 great size and strength of this telescope, to carry the large spectro- 

 scopes of modern times. Through the work of Rowland, whose 

 construction and use of concave and plane gratings has done more 

 than any other one thing to revolutionize spectroscopy, the spectro- 

 scope of the physical laboratory has become an instrument of large 

 proportions and of correspondingly great power. Such an instru- 

 ment may be as much as 40 feet in length, and even if it could be 

 attached to a telescope, the bending of the spectroscope, resulting 

 from its constant changes of position, would render it impossible to 

 obtain sharply defined photographs of spectra. The changes of tem- 

 perature which occur from hour to hour in an open dome, subject to 

 the fluctuations of the outer air, would also interfere with the use 

 of such spectroscopes for photographic work, even if their rigidity 

 were perfect. As a consequence, the solar spectroscopes in use today 

 are in almost every case practically identical with the instruments 

 of a quarter of a century ago. Moreover, many physical instru- 

 ments of recent invention and of extraordinary power are so con- 

 stituted that they cannot be attached to a moving instrument : they 

 must stand absolutely at rest, protected from the most minute dis- 

 turbances, on massive piers, in a constant temperature laboratory. 

 It is evident, therefore, that a telescope for physical work, if it is 

 to be suitable for use with such instruments, must be so constructed 

 as to bring an image of the sun or of a star into a physical 

 laboratory provided with all appliances necessary to protect the 

 delicate instruments from vibrations, from relative flexure of their 

 parts, or, sometimes, from temperature changes of even a few 

 hundredths of a degree, and to meet other requirements demanded 

 in the most refined research. 



The lack of a suitable horizontal telescope made it necessary for 

 Rowland to confine his spectroscopic observations to the light of the 

 sun as a whole, though the use of his powerful spectroscope for a 

 study of the solar details would have yielded results of the greatest 

 importance. 



Other requirements of solar work remain to be mentioned. It has 

 been found that a solar image seven inches in diameter offers de- 

 cided advantages over a two inch image. There can be little doubt 

 that with such atmospheric conditions as exist on Mount Wilson 

 solar images of two feet or possibly even three feet in diameter 



