450 MOssoTTi ON fraunhofer's reticular spectra. 



whence, as A^ = 553*5, the abscissae which on either side of the 

 maximum ordinate are equivalent to the semi-circumference, the 

 lengths of the waves correspond to 553*5 + 184*5, i. e. 



A— , = 369; Aj = 738. 



These two values approximate with sufficient accuracy to those 

 lengths of waves at which the light ceases to be visible. The 

 intensities of light corresponding to these lengths of waves in 

 the points of the normal spectrum would scarcely amount to 

 0'006 of the maximum intensity ; and these points would scarcely 

 differ by ^j'^th of the whole length of the spectrum from the inde- 

 finite limits which are given in Fraunhofer's diagram. If we bear 

 in mind that the observations of this skilful optician were made 

 with great care to assist the eye to discern the faintest traces of 

 light, it may be said that ordinarily the distinct perceptibility of 

 light is produced by waves the length of which extends from 

 369 to 738 millionths of a millimetre, or rather by waves the 

 length of which varies from 1 to 2 ; and that those are most 

 capable of producing the most lively perception, the length of 

 which amounts to 553*5 millionths of a millimetre, or once and 

 a half the length of the smallest wave. 



10. In conclusion, I shall follow the example of Newton in 

 arranging the values of the lengths of the waves corresponding to 

 the principal lines with those of the tones of the diatonic scale: — 



The two first lines of numbers express the relative values of 

 the tones, the lower tone (c) being expressed by unity or the frac- 

 tion j^-gi so that the denominator of the second line represents the 

 lengths of the strings which produce the respective tones. The 

 third line of numbers contains the values of the lengths of the 

 waves corresponding to the principal lines placed above them, 

 expressed in millionths of a millimetre. On comparison, it ap- 

 pears that the lengths of the waves at the lines C, D, H cor- 

 respond with the lengths of the strings of the tones re, mi, si, 

 whilst in the others we only get an approximation. These co- 

 incidences of the dark principal lines, when the proportion is 

 expressed by the direct denominators 4 and 8, and the numerators 



