14 Light and Sound. [Jan., 



Further, the succession of colours in the spectrum is perfectly 

 harmonious to the eye. Their invariable order is red, orange, yellow, 

 green, blue, indigo, violet. Any other arrangement of these colours 

 is less enjoyable. Likewise the succession of notes in the scale is 

 the most agreeable that can be found. The order is C, D, E, F, 

 Gr, A, B.* Any attempt to ascend or descend the entire scale by 

 another order is disagreeable. The order of colours given in the 

 spectrum is exactly the order of luminous wave-lengths, decreasing 

 from red to violet. The order of notes in the scale is also exactly 

 the order of sonorous wave-lengths, decreasing from C to B. 



The interval of wave-lengths embraced between the extreme 

 colours of the visible spectrum is ordinarily as the ratio of 1 : 0*57, 

 corresponding to the interval known as a seventh in music. But 

 the writer is well informed that by proper means further limits can 

 be seen, viz. from what is known as the solar line A to the solar 

 line L.| (See upper figure in Plate : L is not shown.) The wave- 

 length of A is 76, and of L is 38 hundred-thousandths of a milli- 

 metre, or as the ratio of 1 : 0*50, corresponding to the interval of 

 an octave in music, or just the range of the scale. 



Arbitrarily placing C under the colour at the solar line A, viz. 

 a deep brownish red, then the octave higher of C would fall under 

 whatever colour is found at the solar line L, viz. a lavender grey. 

 Now comes this important question, Are the intermediate colours 

 of the spectrum produced by vibrations that bear a definite ratio to 

 the vibrations giving rise to the intermediate notes of the scale] 

 According to our knowledge up to this time, apparently not. 



In an ingenious little work by Dr. Macdonald, before alluded to, 

 an attempt has been made to establish this analogy indirectly; J 

 but if direct comparison fails, it is useless to push the matter 

 farther. Newton himself sought for this analogy between note 

 and colours, but he only found the relative spaces occupied by each 

 colour in the spectrum to be similar to the relative intervals of 

 musical notes. This is, obviously, a false analogy. We must 

 compare wave-lengths of light with wave-lengths of sound ; not, 

 of course, their actual lengths, but the ratio of one to the other. 



Until very recently it has been impossible to do this accurately. 

 New maps of wave-lengths of the different parts of the spectrum 

 have, however, of late appeared. § Let us reduce the newest and 

 best determinations of wave-lengths to a common ratio, and com- 



* The fact that Newton saw seven colours in the spectrum, and there are seven 

 notes in the scale, is only an accident ; the number of colours, or tints, entirely 

 depends on the judgment of the observer. 



f This is on the authority of Mr. Crookes, who has on favourable occasions seen 

 the spectrum extending this length, where a quartz train of lenses and prisms was 

 employed. 



% It is a pity this brochure of Dr. Macdonald 's is so disfigured by its typo- 

 graphy, it is also too speculative and dogmatic. 



§ The most recent by Thalen. ' Transactions of Koyal Society of Upsala,' 3rd 

 Series, vol. vi. ; also ' Annates de Chimie et de Physique,' Oct., 1869. 



