122 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



the latter. So the maximum intensity or illuminating 

 power is, as Mosotti first observed, in the centre, the in- 

 tensity of the light declining symmetrically on each side 

 to the end. 



The Italians have a clear perception, a quick apprecia- 

 tion of the symmetrical and beautiful. When Mosotti 

 first stated this peculiarity of the diffraction spectrum, at 

 a meeting of one of the Italian scientific societies, the an- 

 nouncement was received by the audience with loud ac- 

 clamations of joy. 



I may now describe some of my own studies of these 

 beautiful spectra. 



Kecalling, then, the principle that the wave-length of 

 an incident radiation is proportional to its deviation, let 

 us select upon the paper zone previously described the 

 point where a ray is falling having a wave-length 7866. 

 It is, of course, twice as far from A' as was the violet 

 end <&f the first spectrum, for the selected deviation is 

 double. If we inquire what interpretation the mind 

 will give of a radiation having such a wave-length, an 

 inspection of the zone shows that not only is it visible, 

 but that it is regarded as being of a violet color. 



This is an important fact. We find that a radiation 

 consisting of waves of a given length which is visible 

 will also be visible when the constituent waves are twice 

 that length. And in like manner it might be shown 

 that the same will hold good when they are three, four, 

 five, etc., times that length. Moreover, in all these cases 

 the color impression imparted to the mind will be the 

 same. 



Again, let us select upon the paper zone another point 

 where the wave-length is 15,208. It will have double 

 the deviation of the red end of the first spectrum. Now, 

 agreeably to the foregoing remarks, this point should be 

 visible to the eye, and, for anything that has thus far 



