THE OBJECTIVE PRISM. 



Bv EDWARD C. PICKERING. 

 {Read April 20, 191 2.) 



Three methods may be employed for studying the spectra of 

 the stars. First, the sht spectroscope. This is the method most 

 widely used. The light of the star is concentrated on the slit of 

 the spectroscope, and the linear spectrum widened, if necessary, 

 by a cylindrical lens, or by moving the image of the star. Secondly, 

 by the diffraction grating. As in the first method, the image of 

 the star is concentrated on the slit. But little use has been made 

 of this, and other diffraction methods in studying stellar spectra, 

 owing to the great loss of light. Third, the objective prism. A 

 prism of small angle is placed over the objective of the telescope, 

 and the image of every star in the field is thus spread out into a 

 linear spectrum. Any desired width may be given by allowing the 

 star to traverse the plate slowly, parallel to the edges of the prism. 

 This method cannot well be applied to reflectors, or to other tele- 

 scopes of large size, owing to the size of prism recjuired. Another 

 objection, in the case of reflectors, is that the prism must be placed 

 so far from the mirror that the definition is injured. These diffi- 

 culties may be remedied by the focal plane spectroscope, in which 

 the cone of rays from the star is rendered parallel by a concave 

 lens, then passed through a mirror, and brought to a focus by a 

 convex lens. All the light falling on a large mirror may thus be 

 concentrated into a small space, so that the spectrum of a very 

 faint star may be photographed. But little use has been made of 

 this method, although it appears to have great possibilities. 



The principal advantages of the objective prism are the small 

 loss of light, and the large number of stars which may be photo- 

 graphed simultaneously. Also, that it is not necessary to follow, 

 as -when photographing star charts. The best authorities claim 



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