RELATIONS OF ETHER TO MOLECULES 59 



mode in which they are affected has been illustrated by 

 the action of elastic balls. If a number of them be 

 arranged in a line, if they be of the same size, and the 

 first be struck, it would give up its whole tendency to 

 motion to the next, and would itself come to rest. The 

 second would act in the same manner, as would also the 

 third and the fourth and the fifth, and all in the series. 

 But if two series be placed in a line, the one heavier 

 than the other, and the first of the lighter be struck, the 

 action would be the same as before until the impulse 

 reached the first of the heavier balls, when it would 

 encounter a stronger force of resistance than before, and 

 so, while giving up its whole motion forward, the lighter 

 series would receive an impulse and motion backward. 

 In some such way are rays of light affected, and the 

 energies in the ether made to pass on wholly or in 

 part, to divide and bend according to the resistance 

 encountered. 



Regular reflection. Light falling on a mirror, as of 

 water, mercury, or any polished surface, does not wholly 

 enter it, but is partly bent back in a new direction, or 

 reflected regularly, and partly reflected diffusely, or 

 scattered. The angle, which the line of the reflection 

 of a ray makes with the perpendicular to the surface of 

 a mirror at the point of incidence, is termed the angle of 

 incidence. That which the reflected ray makes with the 

 same perpendicular is termed the angle of reflection. 

 These angles are always in the same plane, and are equal. 

 They can be measured by an instrument, the graduation 

 of which is as perfect as it can be made, and there cannot 

 be discovered the least turning aside from the plane of 

 incidence ; there cannot be discovered the least fraction of 

 a degree of difference in the magnitude of the angles. 

 In passing from point to point, from any point in the 



