REPORT OF COMMITTEE ON OBSERVATORIES 55 



advancement lie in the application of new instruments and methods. 

 Of first importance is the development of the telescope, including : 

 (i; its construction in the horizontal form, especially for work re- 

 quiring a large solar image and the use of spectroscopes and other 

 instruments from the physical laboratory ; and (2) its construction 

 as a large, short focus reflector, equatorially mounted in the coude 

 form, and particularly adapted for the photography of nebulae, the 

 investigation of stellar spectra, and the stud)^ of the heat radiation 

 of the stars. 



Astronomical telescopes are of two kinds — refractors and reflec- 

 tors. The former consist essentially of a lens mounted at the upper 

 end of a tube, which is pointed toward the object to be observed. 

 The lens forms, at the lower end of the tube, an image of the 

 object the size of which varies directly with the length of the 

 tube. Reflecting telescopes, on the other hand, consist of a concave 

 mirror, usually of silvered glass, supported at the lower end of a 

 tube which is open at its upper end. The rays from the object fall 

 upon the mirror, which reflects them back and forms an miage at 

 the upper end of the tube. By means of an additional mirror this 

 image is reflected out at one side of the tube, where it may be ob- 

 served. In both types of telescopes the tubes are pointed directly 

 at the object under observation, and the apparent motion of the 

 object through the heavens is counteracted by a uniform motion of 

 the telescope, produced by clock work. 



The development of reflecting telescopes during the first half of 

 the nineteenth century culminated in the great instrument of Lord 

 Rosse, erected in 1845. The crudenessof the mounting of this tele- 

 scope, due to the lack of suitable engineering facilities, rendered it 

 useless, except for such visual observations as could be made in the 

 absence of a driving clock. Partly for this reason the immense 

 advantages of mirrors over lenses were not discovered, and during 

 the latter part of the century attention was concentrated in large 

 measure on the development of refracting telescopes. These ad- 

 vanced rapidly in size, from the lo-inch telescopes of Frauuhofer at 

 the beginning of the century to the 1 5-inch Harvard telescope ( 1 847 ) , 

 the 36-inch lyick telescope (1888), and, finally (1897), the 40-inch 

 telescope of the Yerkes Observatory. 



(i) But as equatorial telescopes increase in size, it becomes more 

 and more evident that a limit must be set to development in this 

 direction. The driving clock of the Yerkes telescope must move a 

 mass weighing twenty tons with such precision that the image of 



