t 



18(55.] 



Notes and Correspondence. 



183 



rays of a certain wave-length would 

 also absorb, in preference, rays 

 having half that wave-length — in 

 fact, those situated an octave lower 

 in the scale? The relation be- 

 tween the absorptive and radiating 

 powers of bodies for rays of the 

 same refrangibility has been com- 

 pared to the phenomena of con- 

 sonance : and we have in these 

 phenomena a strict parallel for the 

 idea which 1 have just ventured to 

 throw out. Thus, to give an illus- 

 tration,* Count Schaffgotsch, while 

 experimenting on singing names, 

 found that the flame could be set 

 in vibration by sounding not only 

 its fundamental note, but also other 

 notes nearly allied, such as the 

 octave. 



So, again, it might perhaps be 

 found that corresponding to the 

 bright yellow line produced in the 

 spectrum by sodium, a line of great 

 heat exists also among the non- 

 visible rays, at the point where 

 the wave-length is half that of 

 the visible yellow ray ; and absorp- 

 tion-lines may be found to exist in 

 the lower part of the spectrum, in 

 corresponding positions. f If this 

 should be so, we should be able to 

 ascertain the exact length of the 

 different parts of the spectrum, and 



the distance at which a given ray 

 repeats itself. I do not know how 

 far these points may be capable of 

 determination from mathematical 

 data. 



As I have myself no means of 

 working out this subject experi- 

 mentally, I have thought that it 

 might be worth while to suggest 

 these points as worthy of investiga- 

 tion by those who have greater 

 facilities for testing their truth. 



C. Hilton Fagge, M.D. 

 Guy's Hospital, Oct. Ib64. 



P.S. —I have since found that Sir 

 J. P. Herschel* obtained light of a 

 lavender-grey hue by concentrating 

 the ultra-violet rays with a lens. 

 He describes, however, these rays 

 as " lying far beyond the ordinarily 

 visible violet," so that they pro- 

 bably included rays of very different 

 wave-lengths, collected together ; 

 and I do not think that this obser- 

 vation can be regarded as disprov- 

 ing the explanation which I have 

 ventured to suggest, of the occur- 

 rence of the violet rays at the upper 

 end of the spectrum. I cannot find 

 that any writer on the subject has 

 thought of it as a fact requiring to 

 be accounted for. 



On the Vast Antiquity of the Lunar Surface. By James Nasmyth. 



The views I entertain on this sub- 

 ject are that, as a direct consequence 

 of the small mass of the moon, and 

 its comparatively large surface in 

 relation to its mass, the moon must 

 have parted with its original cosmi- 

 cal heat with much greater rapidity 

 than in the case of the earth; con- 

 sequently the moon must have as- 

 sumed a final condition of surface- 



* ' Phil. Mag.' Dec. 1857, p. 542. 



f A tolerably close acoustical analogy 

 for this supposition may be found in the 

 well-known fact that, when those strings 

 of musical instruments which produce 

 the deeper notes are struck, besides the 

 fundamental note, the octave is heard. 



structure ages before the earth had 

 ceased from its original molten con- 

 dition. The moon, in all reasonable 

 probability, never possessed an at- 

 mosphere or water envelope (it cer- 

 tainly has neither now), whde the 

 earth has both. The earth's atmos- 

 phere — and especially its ocean, 

 when it existed in the first instance 

 as a vast vapour envelope, ere the 

 earth had cooled down so as to per- 

 mit the ocean taking up its final 

 position as an ocean — this mighty 

 vapour envelope must have retarded 

 the escape into space of the cosmi- 

 cal heat of the earth millions of 

 * ' Phil. Trans.,' 1840, p. 19. 



* 



