554 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
vibrate. Hence the effect of the two streams together is 'very nearly the same in 
kind as that of one alone, but double in intensity. 
236. The apparent absence of a protecting influence in the less refrangible rays 
seems at first more difficult to account for, but perhaps the following reasoning may 
be thought satisfactory. We ought not to attribute more influence in the direction 
of protection to a second beam of rays of low refrangibility, than in the contrary 
direction to a second beam of rays of high refrangibility. Now if the effect of a 
beam of rays of high refrangibility be to throw 1 per cent, of the molecules into a 
state of vibration, it would be a commensurate effect in a beam of rays of low refran- 
gibility to stop the vibrations of 1 per cent, of the molecules, if they were all vibrating. 
But since only 1 per cent, are actually vibrating, the real protecting effect amounts 
to no more than stopping the vibrations of one molecule in every 10,000, an effect 
which may be regarded as insensible. 
237 . The simple consideration that work cannot be done without the expenditure 
of power, shows that when light incident on a medium gives rise to dispersed light, 
a portion at least of the absorption which the medium is observed to exercise must 
be due to the production of the dispersed light. If the dispersed light really arises 
from molecular disturbances, and for my own part I think it almost beyond a ques- 
tion that it does, it follows that in these cases light is absorbed in consequence of its 
being used up in producing molecular disturbances. But since we must not need- 
lessly multiply the causes of natural phenomena, we are led to attribute the absorp- 
tion of light in all cases to the production or augmentation of molecular disturb- 
ances, unless reason be shown to the contrary. It might seem at first sight that the 
production or non-production of dispersed light establishes at once a broad distinc- 
tion between different kinds of absorption. I do not think that much stress can be 
laid on this distinction. In the first place it may be remarked, that we have no 
reason to suppose that vibrations which are of the same nature as those of light are 
confined to the range of refrangibility that the human eye can take in. If, there- 
fore, no dispersed light be perceived, it does not follow that no invisible rays are 
dispersed. If the incident light belong to the visible part of the spectrum, the 
dispersed rays (if any), being of lower refrangibility than the incident light, can only 
be invisible by having a refrangibility less than that of red light, and would manifest 
themselves solely or mainly by their heating effect. However, though invisible rays 
of this nature are in all probability emitted by the body in consequence of the 
absorption of visible light, we are not bound to suppose that in their mode of emission 
they precisely resemble the visible rays observed in the phenomena of internal disper- 
sion. In most cases, perhaps, they are more nearly analogous to the visible rays 
emitted by solar phosphori. It is possible to conceive, and it seems probable that 
there exist, various degrees of molecular connexion from mere casual juxtaposition 
to the closest chejnical union. A compound molecule may vibrate as a whole, by 
virtue of its connexion with adjacent molecules, or it may vibrate by itself, in the 
