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[Jan. 



NOTES AND CORRESPONDENCE. 



On the probable Existence of the Repetition of Octaves in the Solar Spectrum. 

 By C. Hilton Fagge, M.D., Lecturer on Experimental Philosophy at 

 Guy's Hospital Medical School. 



In his paper, in the October num- 

 ber of the ' Quarterly Journal of 

 Science,' Mr. Balfour Stewart, while 

 comparing the phenomena of ra- 

 diant heat and light with those of 

 sound, remarks that there is not a 

 complete octave in the visible spec- 

 trum — that the most rapid undula- 

 tions which give rise to the sensa- 

 tion of sight have less than double 

 the velocity of those which are least 

 rapid. 



This observation suggested to me 

 the thought, that, probably, just as 

 the ear perceives the identity of one 

 note with another which is an octave 

 higher, so the eye would derive the 

 impression of the same colour from 

 two rays, the wave-length of one of 

 which should be half that of the 

 other, assuming both to possess the 

 power of acting upon the retina. 

 Thus one can conceive, that if our 

 power of vision were less limited, 

 the spectrum, with the actinic and 

 thermal rays, would represent a 

 scale in which the colours would 

 repeat themselves in regular order, 

 just as the notes do in the musical 

 scale. 



We have, 1 think, an evidence 

 that this is the case, in a fact which 

 had often puzzled me ; I refer to 

 the presence of violet beyond the 

 blue in the spectrum, so far from 

 the red, which, by its combination 

 with the blue, would form violet. 

 If the colours be arranged in a 

 circle, the violet forms a gradation 

 between the red and the blue, just 

 as the orange does between the 

 red and the yellow, and the green 

 between the yellow and the blue : 

 and the linear arrangement has 



seemed to me to be an un- 

 natural one. I now believe that the 

 red which thus appears in the violet 

 at the upper extremity of the spec- 

 trum, is not the red which is per- 

 ceived at thelowerextremity,but the 

 red- of an octave higher (so to speak), 

 the lower edge of which happens to 

 fall within the limits in which ethe- 

 real waves are capable of producing 

 the sensation of light. It would 

 follow, that if the actinic rays im- 

 mediately above could be seen, they 

 would appear as red light. It is of 

 course no objection that after pass- 

 ing through certain liquids they 

 appear as blue rays ; for it is ad- 

 mitted that a change in their refran- 

 gibility, and in their wave-length, 

 has then taken place. 



It appears to me that this view 

 suggests several interesting points 

 for inquiry to those who are pro- 

 vided with the means of experi- 

 menting upon this subject. If the 

 colours would repeat themselves in 

 the spectrum were it not for the 

 imperfection of our organs of vision, 

 is it not probable that fixed lines 

 and absorption bands may repeat 

 themselves also ? It is well known 

 that the colour of a surface was found 

 by Melloni not to affect its power 

 of absorbing obscure heat, that is to 

 say, heat of great wave - length. 

 But Professor Tyndall,* with his 

 more delicate apparatus, was able 

 to detect the influence of colour, 

 on the absorption even of obscure 

 rays. Is it not likely that a co- 

 loured surface which absorbs visible 



* ' Lectures on Heat considered as a 

 Mode of Motion,' p. 2S9. 



