MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 123 



been said, it should be interpreted by the mind as red 

 light, its wave-length being twice that of the red of the 

 first spectrum. But it is obvious that here a new con- 

 sideration must enter into account. If this radiation 

 has double the wave-length of the first red, it has triple 

 the wave-length of the first yellow-green. On the prin- 

 ciple just laid down, the mind may interpret it as red 

 light or as yellow-green. Which will it do ? 



Examination of the paper zone, or, better still, through 

 a telescope, shows that the mind adopts both these in- 

 terpretations, and the same principle applying to other 

 wave-lengths, this constitutes what we have spoken of 

 as the overlapping of the second spectrum by the third, 

 etc. At the point here specially considered, both red 

 and yellow-green light are seen. 



From what has here been presented, it follows that 

 the principle considered as established in optics, that 

 to every color there belongs a determinate wave-length, 

 must be modified, since the same color impression will 

 be given to the mind by waves that have twice, thrice, 

 etc., that determinate wave-length. But should the 

 wave-lengths under consideration answer to multiples 

 of that of some other color, the mind will interpret them 

 as being of that color too. 



Moreover, these observations lead us to extend the 

 range of perception of the eye. The prism would lead 

 us to infer that it can only be affected by waves the 

 length of which is between 3933 and 7604. Compari- 

 sons have hence been drawn between the organ of vision 

 and the organ of hearing, to the disparagement of the 

 former. The ear, it is said, can embrace a range of sev- 

 eral octaves, but the eye is influenced by less than one. 

 The grating, however, leads us to reject the restriction, 

 and to place the eye more nearly on an equality with 

 the ear. 



