OPTICAL l'i;<)|'i.l!Tli:> OF ' IM 3TALS 



16 



telegraphy They both travel with the same velocity of 185,400 

 Illilrs per second, and may l>e polari/.ed, reflected, dct'i'iicti-d. 

 and refracted. The wireless waves are very lartie, while those 

 waves which our eyes are aMe to detect as light arc very 

 small. The range of our eye as a detector is limited to tho>e 

 waves which fall within the colored spectrum ; but that waves, both 

 smaller, as the ultra-violet waves, and larger, as t he infra-red waves, 

 do exist we know from other detectors, and our eyes are not able to 

 recognize these waves as light. The ether pervades all space, both 

 the interstellar and the intermolecular, and penetrates even within 

 the atom itself, filling the space between the electrons which com- 

 pose it. The interatomic space is probably as accessible to the 

 ether as the space within a stack of bird cages is to the air, and yet 

 light travels faster in a vacuum than in space filled by a gas or a 

 transparent solid. It is through this modification of the velocity 

 and vibration of the light wave, as it passes through a substance, 

 that the optical properties of any particular crystal become appar- 

 ent. Light may be considered as transmitted through a given 

 medium by means of waves set up in the ether. The periodic 

 changes which constitute these waves take place at right angles 

 to the line of propagation, and in this respect they are known as 

 transverse waves vibrating back and forth in all planes across the 

 line of direction of transmission. The ray is a term conveniently 

 used to denote the direction along which the wave advances. 



As an illustration of the terminology of wave and wave motion it 

 is best to select, as an example, one in which there is possibly no 

 imagination required, as is the case of the wave motion on the sur- 

 face of water. 



FIG. 30!). 



In Fig. 309, the position of any particle of water, as a, on the sur- 

 face will determine the wave surface at that point ; as a falls to- 

 ward the arrow, the water surface falls and the wave passes on until 

 a reaches a maximum depression x, when the valley of the wave z 

 is formed ; then a rises until it reaches a maximum position above 



