VIII.] 



METHODS OF COMPARISON. 



103 



eollimator, which it enters side by side with the direct light 

 from the centre of the disc. 



FIG. 39. a, right-angled prism on which the image of the limb falls ; b, right- 

 angled prism which receives light from a ; c, slit through which centre of sun 

 is observed ; d, screw to adjust width of slit ; e, screw to adjust distance of a 

 from b, according to the semi-diameter of solar image employed. 



An arrangement suggested by Mr. Hastings for accomplishing 

 the same result is shown in Fig. 40. The light from the edge 

 of the sun reaches the slit after two total reflections from the 

 faces of the prism, as shown in the figure, while that from the 

 centre enters the uncovered part of the slit directly. 



FIG. 40. Mr. Hastings's arrangement for comparison of spectra. 

 1 1', diameter of sun's image ; p, totally reflecting prism. 



In spite, however, of the new apparatus, it was very difficult 

 to see any special effect at the limb. 



