374 THIRD REPORT — 1833. 



tude to propagate vibrations of some determinate degree of 

 frequency, than of a greater or less degree, as being in unison, 

 or an approach to unison, with that in which they themselves 

 would vibrate if existing alone, and agitated by an external im- 

 pulse. But this greater disposition to propagate some vibra- 

 tions does not render them incapable of transmitting others. 

 In illustration of this position Sir John Herschel exhibited an 

 experiment where the column of air in a closed pipe, being main- 

 tained in a state of forced vibration by two tuning-forks held 

 over it, differing materially in pitch, yielded at the same instant 

 both their sounds, being actually out of unison with itself, and 

 uttering distinct beats*. It is on this principle oi forced vibra- 

 tions that, according to the view here taken of the subject, the 

 pheenomena of absorption, or, as it should rather be called, of 

 obstructed transmission, depend. The degree of obstruction 

 which an undulation experiences from a medium will be in pro- 

 portion to the number and extent of its subdivisions, in travers- 

 ing the molecular systems of which that medium consists, and 

 the latter will be greater the more completely out of unison 

 those systems happen to be with the vibrations thus forced 

 through them. To assign the law of degradation may be dif- 

 ficult, but it is easy to satisfy ourselves that it may depend on 

 relations quite sufficiently varied and different in differently 

 constituted molecules, to give rise to all the apparently capri- 

 cious phaenomena of absorptionf . 



On the Dispersive Powers of the Media of the Eye, in con- 

 nexion with its Achromatism. By the Rev. Baden Powell, 

 M.A., F.R.S., Sav. Prof, of Geometry, Oxford. 



The principal experimental results bearing on the question 

 of the achromatism of the eye are as follows : 



At distances less than the limit of distinct vision the image 

 of a luminous point is coloured ; or if a small opake object be 

 interposed, its shadow is coloured. 



To near-sighted eyes, a small bright object on a dark ground 

 appears edged with colour, and the effect is rendered more con- 

 spicuous by using a blue glass, which allows the extreme rays 

 of the spectrum to pass, and stops or weakens the middle rays. 



* This experiment, which was published by Sir John Herschel in Yas Essay on 

 Sound, was then supposed by him to be new. It appears, however, to have been 

 previously made by Mr. Wheatstone. See a very ingenious paper by that 

 gentleman, " On the Resonance of Columns of Air," Quarterly Journal of 

 Science, N.S. No. 5. 



t The paper, of which the above is an abstract, has since been communicated to 

 the London and Edinburgh Philosophical Magazine and Journal, vol. iii. p. 401. 



