THE FUNCTION OF LARGE TELESCOPES. 131 



Such small members of the solar system are by no means tbe only 

 feebly luminous objects which great telescopes have brought to light. 

 Faint stars in the close proximity of bright ones are usually beyond 

 the reach of small telescopes. Thus the companion of Sirius was not 

 seen until 1862, when the late Alvan Gr. Clark encountered it in his 

 tests of the 18-inch objective now at the Dearborn Observatory, which 

 was the largest glass that had been constructed up to that time. . The 

 small companion to Procyon, discovered not long ago by Professor 

 Schaeberle with the Lick telescope, is another object of the same type. 

 These are conspicuous examples of that great class of objects known 

 as double stars, which consist of two stars revolving about their com- 

 mon center of gravity. From the third advantage of large instruments, 

 to which reference has already been made, it will be seen that they are 

 peculiarly adapted for the investigation of these binary systems, not 

 only because of their power to show faint objects in the neighborhood 

 of brighter ones, but also on account of their capacity to separate two 

 closely adjacent stars which in a smaller instrument would be seen as 

 one. Thanks to this property, many interesting binary systems whose 

 components are exceedingly close together have been found by Pro- 

 fessor Burnham with the Lick telescope, and, although he has devoted 

 no special attention to a search for such objects, Professor Barnard lias 

 already encountered several of them in his work with the Yerkes 

 refractor. From what the spectroscope has taught us of binary sys- 

 tems, we have every reason to believe that telescopes may go on 

 increasing in aperture almost indefinitely without ever arriving at the 

 possibility of separating into their component parts all existing double 

 stars. As has been stated, the Yerkes telescope can show as distinct 

 objects stars which are no farther apart than 0.12" of arc, and on 

 account of the elongation of the image a double star whose compo- 

 nents are only 0.1" apart can be distinguished from a single star. But 

 there undoubtedly exist stars far closer together than this, some of 

 which can be separated by an aperture of not less than 40 feet. 



There has been much discussion in recent years regarding the rela- 

 tive advantage of large and small telescopes for observations of the 

 markings on planets. I do not propose to enter into the details of this 

 discussion, partly because my own investigations are primarily con- 

 cerned with observations of another nature, and thus have not espe- 

 cially qualified me to form an opinion on this point, and partly on 

 account of the fact that additional arguments in favor of large instru- 

 ments would serve little purpose. It seems to me only necessary for an 

 unprejudiced person to examine a planet first with a small telescope of 

 from 5 to 15 inches aperture, and then to look at the same object with 

 an instrument of 3G or 40 inches aperture, under identical atmospheric 

 conditions. When the seeing is distinctly bad, that is, when the 

 atmosphere is in so disturbed a state that the images are blurred and 

 unsteady, the smaller instrument will assuredly show all that can be 



