SUNLIGHT, SEA, AND SKY. 57 



produce waves advancing in another will be familiar to all of you who 

 have watched the movement of a cork floating on the sea. You will 

 have noticed that the cork has simply moved up and down, or nearly 

 60, while the waves have passed, as it were, under it, along the surface 

 of the water. 



Now, in order to make clearer to our minds how this wave-motion 

 is produced, I will throw the electric light upon a machine devised for 

 the purpose. You now see a horizontal row of knobs. As the slider 

 is pushed in, the knobs at one end begin to rise in succession until each 

 has in turn attained its greatest elevation. Immediately after reach- 

 ing its highest position it begins to descend; so that the knobs first 

 rise and then fall in regular succession, and continue to rise and fall 

 in the same manner so lonsr as the motion is continued. Each of the 

 knobs, beginning from number one, is thus successively at the highest 

 position, while at the same moment those immediately before and be- 

 hind it are at lower positions. And as the knob which is at the highest 

 position represents what we call the crest of the wave, the crest will 

 pass successively along all the knobs, beginning from the first. Thus 

 the waves are transmitted along the line, While the vibrations take 

 place across it. If the line of knobs represent the direction of a ray, 

 their motions will represent the vibrations and waves to which the 

 light is supposed to be due. In ordinary light these vibrations may 

 take place in any directions perpendicular to the ray ; and the effect 

 of the crystal of which the Nicol is made, is to restrict these vibrations 

 to a particular direction. In the arrangement now before you the first 

 Nicol causes the vibrations to be altogether horizontal. When the 

 second Nicol is placed similarly to the first, it will obviously have no 

 further effect upon the light ; but if it be turned through an angle, it 

 will transmit only vibrations inclined to the horizontal at that angle ; 

 that is, only such part of the original horizontal vibrations as can be 

 brought into the inclined direction ; in other words, it will transmit 

 only part of the light. And as the inclination is increased the part of 

 the light transmitted will diminish, until, when the second Nicol is in 

 a position to transmit only vertical vibrations (i. e., when it has turned 

 through a right angle), the light will vanish. Such is an explanation 

 of this fundamental experiment in polarization on the principle of what 

 is called the Wave Theory of Light ; and I have ventured to give it in 

 some detail, because it is the key to all others, and forms a starting- 

 point for any who may desire to go further in the subject ; and it is a 

 remarkable feature in this Wave Theory of Light that the results of 

 many other experimental combinations, to some of which we will now 

 proceed, might be predicted upon the principles already laid down. 



If a plate of crystal, such as selenite, be placed between the two 

 Nicols, and turned round in its own plane, it will be found that in 

 certain positions at right angles to one another no effect is produced. 

 These may be called neutral positions. In all other positions the field 



