As soon as the aetherial atom which forms the spherical aetherial envelope has reached the limit of its expansion, it seeks to recover its former position because of its elasticity, with the result that the whole envelope contracts again, and arrives at its original position in space ready to accept motion again and transmit it onwards in the same manner as before. Thus, by the acceptance of an atomic and gravitating Aether, we may form a physical conception of one of the greatest problems in optical phenomena, viz. the transverse vibration of light which always takes place in the wave front, and across the line of propagation. Whether this explanation is exactly correct in detail, or not, I am convinced that the true physical explanation of the problem is to be found in an atomic and gravitating Aether, as hitherto a frictionless Aether has failed even to suggest to any scientist how such a transverse vibration can take place. ART. 72. _Reflection and Refraction._--A ray or wave of light is said to be reflected when it meets with an obstacle which opposes its free passage and turns it back. We have illustrations of this law of reflection in the case of water waves striking against a breakwater, or a sound wave striking against the wall of a room. In either case the wave is turned back, and reflection is the result. A ray or a wave of light is said to be refracted when, in passing from one medium into another, it is turned from the straight path in which it was going before it entered the refracting medium. An illustration of the refraction of light is to be found in the case of the glass lens, so often used to converge the light waves into one focus. We have up to the present dealt with only two theories of light, the Corpuscular theory and the Undulatory or Wave theory. We have seen how both harmonize with Huyghens' principle, and the question arises as to whether both can be made to harmonize with the phenomena of reflection and refraction. In the Corpuscular theory we have luminous particles emitted by luminous bodies. These particles we have learned are practically synonymous with our aetherial atoms. In the Wave theory it is impossible to conceive of a wave without conceiving of particles which transmit the wave; even Huyghens refers to particles of Aether, and so does Tyndall in his _Notes on Light_. In the Electro-magnetic theory of light we have again to think of atoms, which are termed electrons by Dr. Larmor and Sir William Crookes; while Professor J. J. Thompson calls them corpuscles.