STELLAR EVOLUTION. 299 



horizon. In order that the observing slit may be directed to any part 

 of the sky the dome, 90 feet in diameter, is mounted on wheels and can 

 be 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 conjunction 

 with appropriate instruments, which are attached to the lower 

 end of the tube near the point where the image is formed. I 

 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 observations, and its maker, the late Alvan 

 Ct. Clark, had no idea that it would ever be employed for photog- 

 raphy. Without dwelling upon the distinguishing features of visual and 

 photograpliic lenses I may say that the former is so designed by the 

 optician as to unite into an image those rays of light, particularly the 

 yellow and the green, to which the eye is most sensitive. With the only 

 varieties of optical glass which can be obtained in large pieces it is im- 

 possible to unite in a single clearly defined image all of the red, the 

 yellow, the green, the blue, and the violet rays which reach us from a 

 star. Therefore when the optician decides to produce an image most 

 suitable for eye observations he deliberately discards the blue and violet 

 rays, simply because they are less important to the eye than the yellow 

 and green rays. For this reason the image of a star produced by a large 

 refracting telescope is surrounded by a blue halo containing the rays 

 discarded by the optician. These very rays, however, are the ones to 

 which the ordinary photographic plate is most sensitive; hence in a 

 photographic telescope the blue and violet rays are united, while the 

 yellow and green rays are discarded. 



The 40-inch telescope is of the first type, constructed primarily for 

 visual observations. In order to adapt it for photography Mr. G. W. 

 Eitchey of the observatory staff simply places before the (isochromatic) 

 plate a thin screen of yellow glass, which cuts out the blue rays, but 

 allows the yellow and green rays to pass. As isochromatic plates are 

 sensitive to yellow and green light there is no difficulty in securing an 

 image with the rays wliich the object-glass unites into a perfect image. 

 During the entire time of the exposure a star which lies just outside the 

 ^ region to be photographed is observed through an eye-piece magnifying 

 ],000 diameters. This eye-piece is attached to the frame which 

 carries the photographic plate, and is susceptible of motion in 

 two directions at right angles to each other. In the center of 

 the eve-piece are two very fine cross-hairs of spider web illuminated 

 by a small incandescent lamp. If the observer notices that through 

 some slight irregularity in the motion of the telescope, or through 

 some change of refraction in the earth's atmosphere, the star image is 



