54 Mr. Jamefi Rice on 



along the whole train ; and also can we obtain two or more 

 sources of such light as nicely adjusted in phase to one another 

 as the needles of a ripple-tank. The answer is in the affirmative. 

 The light emitted from incandescent sodium, lithium, strontium, 

 to choose a few familiar substances, the light emitted from tubes 

 containing traces of any gas and electrically excited by a coil 

 possesses in a high degree those elements of simplicity required. 

 From such sources we can obtain beams of almost homogeneous 

 or " monochromatic " light, i.e., light of definite wave length or 

 frequency. Square across the path of such a beam we can put 

 what is called a grating, consisting of a series of fine, equally 

 spaced, parallel lines ruled by a diamond point on a piece of 

 glass. Such lines serve to scatter and absorb some of the 

 luminous energy of the advancing beam ; the remainder of 

 the energy will emerge through the clear pieces of glass lietween 

 the lines — the spaces as they are called, These spaces, as a 

 matter of fact, serve as sources of light possessing that nice 

 adjustment of phase which was postulated above as an experimental 

 necessity. Instead of two such sources only, we generally have 

 several thousands. The essentials of the occurrence are, however, 

 still i>reserved ; it is found that the light energy is directed not 

 merely along its original direction, but also along several other 

 well defined directions. The slide shows you a crude picture 

 of some of the energy being directed along one direction ; it is 

 collected by the lens and focussed to a fine line. The slide 

 shows you only one of these lines corresponding to one direction 

 of energy flow. There are in general several such line images, 

 and their distance apart serves to determine the angular separation 

 of the various paths of the energy, and this, as you will remember, 

 determines the wave length, once we know the distance apart 

 of the successive spaces of the grating. In actual practice the 

 line image is projected on the vertical cross wire in a telescope 

 eye-piece, which permits of a very fine adjustment of the 

 telescope and a very precise determination of the divergence of 

 each energy stream from the original direction of the light. As 



