Radiant Forces. 139 



along an immense space without a loss of power ; and a suf- 

 ficient length of space might at last) bring it to a state of rest, 

 which would be equivalent to destroying its character as light. 

 Indeed, that character would cease long before rest was 

 attained, because no eye, constructed upon the principles of 

 human eyes, could be impressed with the sensation of light, 

 unless the velocity of the ether vibrations were sufficiently great. 

 As soon as they declined below a certain quickness, they would 

 cease to excite the sensation of light, and vision can take no 

 cognizance of them. 



If light were regarded according to the Newtonian theory 

 of its consisting in an emission of particles, mathematicians 

 would have to calculate how closely it would be possible for 

 independent light-streams to be packed together ; and when 

 assigning to any two of them an infinitely small divergence, 

 to compute how far they must travel before the divergence 

 would increase to such an extent, that they must pass on 

 either side of an object like our earth or one of the planets. 

 It would be interesting to know, according to this theory, what 

 proportion the truly parallel rays from the sun would bear to 

 the diverging rays which strike upon the various members of 

 his system. 



The emission theory is, however, generally abandoned, 

 and the term "ray" needs special explanation when used in 

 connection with the undulatory theory of light. If we throw 

 a stone in the centre of a still pond, we notice a series of con- 

 centric hollows and ridges, spreading out wider and wider, and 

 growing fainter and fainter, until, if the pond be large enough, 

 they gradually vanish in the smooth surface. If the pond is 

 small in proportion to the size of the stone and the force with 

 which it strikes the water, strong waves continue up to its 

 banks, and are reflected back again, producing noticeable 

 interferences with advancing waves. Now in these actions, or 

 in the first of them, the spreading of waves from the centre to 

 the circumference of the pond, we do not notice any analogy 

 to " rays ;" but if we consider that every hollow and ridge 

 cousists of multitudes of water particles in a state of oscillation, 

 the term "ray" might be applied to each diverging series of 

 particles, arranged in sets, one in front of the other, which 

 oscillate or move up and down in the same vertical plane, and 

 which all owe their motion to a transmission of "force from 

 one of the originally moving particles. From the cohesion of 

 the water particles there are no lateral gaps between these 

 water "rays." Each particle of water imparts its own motion 

 to its neighbours, and as the waves increase in size, the number 

 of particles in motion becomes greater. The effect of this 

 constant addition of particles to the wave system is, that the 



