248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



have as if deformed, bent back upon itself, so that instead of having 

 a simple wave we will have to deal with a superposition of simple 

 waves. The state of oscillation at any point depends upon the re-, 

 suit of the superposition of the various waves, an effect sometimes 

 additive, at times subtractive. If the simple waves are additive, or 

 as we may say, are in the same phase, a very strong oscillation re- 

 sults; if, on the other hand, the waves oppose each other, or, as we 

 may say, there is an opposition of phase, then the resultant vibration 

 will be very small or even possibly zero. Now it can be shown that 

 the existence at a point of concordance or discordance of phase de- 

 pends essentially upon the wave length of the wave wiiich happens 

 to strike the obstacle. Resuming, the presence of an obstacle dis- 

 turbs the passage of a w^ave, causing the appearance of a complicated 

 distribution of vibration intensities depending essentially on the 

 wave length of the original wave. There result the phenomena of 

 interference and diffraction. 



If we adopt this idea that light is made up of waves, we are 

 led to predict that if obstacles hinder the free passage of the bundle 

 of rays, then the phenomena of interference and diffraction will 

 make their appearance. Within the region of interference the dis- 

 tribution of light will be complicated but we can easily predict 

 it from the wave length of the light, that is to say, from the color 

 of light employed. Young and Fresnel showed that light exhibits 

 the phenomena of interference and diffraction. Fresnel showed fur- 

 ther that the conception of light waves sufficed to explain in every 

 detail all the observed phenomena of interference and diffraction. 

 On the other hand, the theory which considers light as being simple 

 moving corpuscles can not lead to the conception of wave lengths 

 and is wholly incapable of explaining interference. Following 

 Fresnel, and during the whole of the last century, the undulatory 

 conception of light was admitted without opposition. 



You know there exist divers kinds of simple light, each corre- 

 sponding to a definite color. The white light given out by an 

 incandescent body, an electric light, for instance, is made up of a 

 continuous sequence of simple lights of which the colors vary pro- 

 gressively by imperceptible gradations from violet to red, forming 

 what we call the spectrum. The wave theory of light naturally 

 led to the designation of the quality of each component of the spec- 

 trum by its wave length; that is, there exists a correspondence 

 between wave length and color. Since interference phenomena de- 

 pend upon the wave length, they give us a means of measuring the 

 wave length corresponding to each color of the spectrum. And 

 so we have been able to determine that the wave length varies, 

 increasing continuously from the violet end of the spectrum where 



