410 Sir John F. W. Herschel on the Absorption 



ofligbt through gross media to be performed, so as to bring 

 the absorptive phenomena within the wording of this principle, 

 is, to regard such media as consisting of innumerable distinct 

 vibrating parcels of molecules, each of which parcels, with the 

 portion of the luminiferous aether included within it, (with 

 which it is connected, perhaps, by some ties of a more intimate 

 nature than mere juxtaposition,) constitute a distinct compound 

 vibrating system, in which parts differently elastic are inti- 

 mately united and made to influence each other's motions. 

 Of such systems in acoustics we have no want of examples — 

 in membranes stretched on rigid frames, in cavities stuffed 

 with fibrous or pulverulent substances, in mixed gases, or in 

 systems of elastic laminae, such as boards, sheets of glass, reeds, 

 tuning forks, &c, each having a distinct pitch of its own, and 

 all connected by some common bond of union. In all such 

 systems the whole will be maintained in forced vibration so 

 long as the exciting cause continues in action, but the several 

 constituents, regarded separately, will assume, under that in- 

 fluence, widely different amplitudes of oscillation, those as- 

 suming the greatest whose pitch taken singly is nearest to 

 coincidence with that of the exciting vibrations. Everybody 

 is familiar with the tremor which some particular board in 

 a floor will assume at the sound of some particular note of an 

 organ ; but when that note is not sounded, it is sufficiently 

 apparent that the board is no less occupied in performing its 

 dynamical office of transmitting to the soil below, or disper- 

 sing through its own substance and the contiguous bodies, 

 the motion which the oscillation of the air above is continually 

 imparting to it. 



As we know nothing of the actual forms and intimate nature 

 of the gross molecules of material bodies, it is open to us to 

 assume the existence, in one and the same medium, of any va- 

 riety of them which may suit the explanation of phaenomena. 

 There is no necessity to suppose the luminiferous molecules of 

 gross bodies to be identical with their ultimate chemical 

 atoms. I should rather incline to consider them as minute 

 groups, each composed of innumerable such atoms ; and it 

 may be that in what are called un crystallized media, the axes 

 or lines of symmetry of these groups may have no particular 

 direction, or rather all possible directions, or the groups them- 

 selves may be unsymmetrical. Such a disposition of things 

 would correspond with a uniform law of absorption, independ- 

 ent of the direction of the transmitted ray, while in crystallized 

 media a uniformity of constitution and position of these ele- 

 mentary groups, or rather of the cells or other combinations 

 which they may be regarded as forming with the interfused 

 Aether, may be readily supposed to draw with it differences in 



