MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 



there be placed behind the grating a convex lens, or, still 

 better, if the lens be the objective of a telescope. 



Now the eye can only be impressed by special radia- 

 tions consisting of waves of a determinate length. Its 

 vision is limited to those that impart to it a sensation of 

 red on one hand, and of violet on the other. To all oth- 

 ers it is blind. Then, though the whole paper zone is 

 receiving radiations of every kind, the eye selects out 

 only those that it can perceive, and, as a result, sees in 

 the four quadrants, Q', Q", Q'", Q"", those only for which 

 it is fitted. 



It follows, therefore, that at A' there is a white image 

 of the slit S, and to the right and left of this there are 

 equal spaces, p, p', completely dark. Beyond, and sym- 

 metrically on each side, there is a series of spectra, v r, 

 v r /, v" r" , etc., of which the violet ends are nearest A', 

 and the red ends most distant. These spectra are des- 

 ignated respectively as being of the 1st, 2d, 3d, etc., order. 

 On each side the 1st spectrum is separated from the 2d 

 by an obscure space, r v', which is shorter than the first 

 dark spaces, p,p' , and the red end of the 2d spectrum is 

 overlapped by the violet of the 3d. In like manner the 

 3d is overlapped by the 4th, etc. If the intromitted ray 

 be of sunlight, and a convex lens or small telescope be 

 used, the dark Fraunhofer lines are seen in these spectra. 



Such are the results seen in the quadrants Q"', Q x/// , 

 from the light transmitted through the grating. In the 

 quadrants Q', Q", exactly the same train of phenomena 

 will be discovered dark spaces and spectra, the latter 

 having their violet ends nearest to A, and the overlap- 

 ping of successive ones taking place in the manner above 

 described. 



Since the results are thus symmetrical in all the four 

 quadrants, it is sufficient to select one of them for de- 

 tailed examination. Let it be the quadrant Q"". 



