STELLAR EVOLUTION. 153 



the photoo-raphs theniselves (compiiiv PI. Ill with PI. TV). The sniiiU 

 chi.ster, which in reality contains several thousands of stars, is resoh (hI 

 l)}^ Mr. Ritehey's photograph, taken with the larye telescope, into all 

 its constituent parts, stars less than one secoml of arc apart ))einu' 

 clearly separated on this ereat scale. 



Having seen this illustration of the superior power of the large 

 telescope you may periiaps lie interested to become more closely 

 ac(juainted with the instrument itself (PI. V). The great weight of 

 the 4()-inch lens, amounting with its cell to half a ton, requires that 

 the tube which supports it. here taking the place of the camera box of 

 the previous instrument, shall be of immense rigidity and strength. 

 This tube, H4 feet in hmgth, is supported at its middle point by the 

 declination axis, which in its turn is carried l)y tlie })olaraxis, adjusted 

 to accurate parallelism with the axis of the earth. By means of driv- 

 ing mechanism in the upper section of the iron column the whole 

 instrument is turned about this polar axis at such a rate that it would 

 complete one revolution in twenty-four hours. Although the mov- 

 ing parts weigh" over 'H) tons, thc^ telescope can be directed to an}" 

 part of the sky by hand, but this operation is much facilitated by the 

 us(> of electric motors provided for th(^ purpose. When once directed 

 toward the object to be observed it will frequently happen that the 

 lower end of the telescope is far out of reach above the o))server's 

 head. For this I'eason the entire floor of the observing room, 75 feet 

 in diameter, is constructed like an electric elevator, which, by throw- 

 ing a switch, can he made to rise or fall through a distance of 23 feet. 

 Thus th<^ lower end of the telescope is rendered accessible even for 

 o])jects near the horizon. In order that the o})serving slit may be 

 directed to an}" part of the sky the dome, 1»() fe(^t in diameter, is 

 mounted on wheels and can })e turned to any desired position by means 

 of an electric motor controlled from the rising-floor. 



The telescope is used for a great variety of purposes in conjiuiction 

 with appropriate instruments, which are attached to the lower end of 

 the tu})e near the point where the image is formed. 1 have already 

 shown a photograph of a star cluster taken with this telescope, but 

 without describing the process of making it. As a matter of fact the 

 object-glass of the 40-inch telescope was designed for visual observa- 

 tions, and its maker, the late Alvan (1. Clark, had no idea that it 

 would ever be employed for photogra})hy. Without dwelling upon 

 the distinguishing features of visual and photographic lenses 1 may 

 say that the former is so (h'signed l)v the op'"ician as to unit(^ into an 

 imago those rays of light, particularly tlu^ yellow and the green, to 

 which the eye is most sensitive. With the only \ arieties of optical glass 

 which can be obtained in large pieces it is impossible to unite in a sin- 

 gle clearly deflned image all of the red, the yellow, the green, the blue, 

 and the violet rg,ys which i-each us from a star. Therefore when the 



