158 ELEMENTS OF LABORATORY WORK 



moved along it to distances measurable by the verniers they 

 carry. The first stand carries the eye-piece with a vertical 

 cross-wire. This may be moved horizontally by a micrometer 

 screw. A scale and vernier shows the displacement. The 

 middle upright holds the bi-prism and the further one carries 

 the slit. These two are capable of horizontal and vertical 

 movements. The centres of the slit, bi-prism, and eye-piece 

 should be in a straight line parallel to the bench-scale. The 

 edge of the bi-prism should be vertical and pass through this 

 straight line. The cross-wire and slit should coincide with the 

 edge of the bi-prism, and the cross- wire must move accurately 

 at right angles to this line, in order to measure the distance 

 apart of the interference lines. 



101. Explanation of Interference. The most complete ex- 

 planation of the previously observed properties of light is 

 given by the wave theory, according to which light consists of 

 a vibratory motion of a universal medium, called the ether. The 

 ether allows this vibratory motion, when once it is set up, to 

 be propagated with very great rapidity and with undimmished 

 intensity in all directions. Ether is present in matter as well 

 as in. space, occupying the spaces between the particles. Light 

 may therefore pass through some kinds of matter ; others are 

 only slightly penetrated, the motion of the ether being pro- 

 bably obstructed or changed by the particles of matter. We 

 are unable to observe ether directly ; we are not conscious of 

 its existence as we are of matter \ yet, by assuming the exist- 

 ence of such a medium, we are able to put forward a simple 

 explanation of the most varied optical phenomena. 



We are not concerned, at this stage, with the exact nature 

 of the vibratory movement constituting light. We have 

 simply to consider that this vibratory movement is rapidly 

 propagated without any portion of the ether undergoing more 

 than a small excursion from its position of rest, just as a 

 wave of water travels far and wide, while a body floating on 

 the surface of the water shows by its motion up and down that 

 the wave travels, but not the water. The movement travels 

 on, leaving the ether behind it unchanged. If it reaches our 

 eyes with sufficient intensity, we are conscious of light. If it 



