TRANSACTIONS OF THE SECTIONS. 551 



(I.) the great number of the dark Hnes in many kinds of Hght, 

 which would appear to make a very complex structure of me- 

 dia necessary, (2.) the difficulty of conceiving that, on such an 

 hypothesis, the absorptive properties of media should be the 

 same in all directions. Both these objections seem to be much 

 diluted by the following considerations : 



1. A combination of channels of vibration which would 

 destroy any rate of vibration, would also destroy all the har- 

 monics of that rate, if the vibrating body were a line. If the 

 fundamental rate were much slower than those which were no- 

 ticed by the senses, this consideration would give many more 

 vibrating rates near each other. Thus, if the fundamental 

 rate were a million undulations a second, we should have a 

 dark line for every multiple of this; and, therefore, since red 

 light makes 458,000,000,000,000 vibrations, and violet light 

 727,000,000,000,000 vibrations, per second, we should have 

 270,000,000 lines in the spectrum on this account only. 



But it is to be observed that the vibrating masses of the 

 aether are not lines. The experiments on vibrating plates have 

 shown that the harmonics of plates are more numerous and va- 

 ried than those of lines, as theory also shows. But the vibra- 

 ting masses of aether are solid spaces, and the way in which 

 they may be divided by nodal surfaces into portions vibrating 

 isochronously will be still more various ; so that in this way the 

 rates of vibration for which the vibration is extinguished may 

 become as numerous as any observations can require. 



2. If we conceive with Sir John Herschel a medium which 

 will not transmit vibrations except through certain canals, these 

 canals must have a determinate direction ; and therefore such 

 a constitution of diaphanous bodies would give different pro- 

 portions in different directions. But let a medium consist of 

 certain particles regularly distributed, the intervening space 

 being filled by a medium capable of vibration. Let it be sup- 

 posed, also, that each vibration, on reaching a medium so dis- 

 posed, proceeds in part directly, and in part by the indirect 

 routes which go round some of the particles and rejoin the di- 

 rect course. We have thus combinations of ramifying and re- 

 uniting paths, which, though very complex in 6ach dii-ection, 

 are the same in different directions, in consequence of the re- 

 gular distribution of the particles. If the distribution, though 

 regular, have a reference to certain axes, as in many crystals, 

 the phaenomena of absorption may be different in different di- 

 rections with regard to these axes. 



In this way the theory of ramifying canals comes to coincide 

 with the theory of vibrations, of which parts are differently re- 



