ELECTRIC AND HEAT WAVES NICHOLS AND TEAR 183 



indicate differences in the length of path of the two beams reflected 

 by the two mirrors M 15 M 2 . This path difference equals twice the dis- 

 placement of the movable mirror. 



A more complete knowledge of the form of wave trains sent out 

 by the oscillator is obtained by the newly designed reflecting echelon 

 analyzer shown in Plate 2, Figure 2. This instrument is in the 

 form of a staircase made of 10 exactly equal brass blocks, the lower 

 back edges of which rest against a slanting plate of glass. The 

 tilt of this glass plate and consequently the width of tread of the 

 steps can be controlled and measured by the micrometer screw seen 

 at the top of the apparatus. The way a single wave from the oscil- 

 lator is divided up after reflection from the rises of the steps is 

 shown diagrammatically in Figure 4, B. A shows the outline of 

 the steps and C is the wave form obtained when the short individual 

 wave trains are brought together again at the receiver. By vary- 

 ing the width of the steps, this instrument gives a complete analysis 

 of the wave trains, showing both wave length and form. 



The simplicity of the principles involved in these experiments 

 and the homely character of the apparatus used may easily conceal 

 the almost vanishing smallness of the quantities measured and the 

 great experimental difficulties which had to be overcome. 



The extended electric wave spectrum. — With the instruments and 

 methods indicated we have not only artificially manufactured and 

 measured electric waves 220pi (one one-hundredth inch) long but 

 have used our electric wave receiver in two different forms to detect 

 and remeasure the 324jx (one seventy-fifth inch) heat waves obtained 

 from hot bodies. Thus, for the first time, waves from hot bodies 

 and artificially generated electric waves of the same length have 

 been obtained, compared, and found identical in character. These 

 experiments tkus supply the last link in a long chain of experi- 

 mental evidence connecting light with electric waves and furnish 

 a final proof of Maxwell's electromagnetic theory of light. 



That not only radiation emitted by highly heated bodies, including 

 infra-red, visible, and ultra-violet rays are electric waves, but so also 

 are X ray and gamma rays from radium, can no longer be questioned. 

 Thus the true electric wave spectrum is expanded by an enormous 

 extent, for a 20,000-meter radio wave is over 20,000,000 billion times 

 as long as a short gamma or X ray. 



Of many things thus brought to notice, nothing is more surpris- 

 ing than the very narrow limitations of the eye in perceiving electric 

 waves as light. Our entire range of vision from the farthest violet 

 to the deepest red is only from a wave length of about 0.39^ to 

 0.78/x. It is not possible to show such small detail and such vast 

 extent as is embodied in the electric wave spectrum by any chart 



