THE CENTURY'S PROGRESS IN PHYSICS 



ity of insight which we call genius, he saw of a sudden 

 that the phenomena could be explained by supposing 

 that when rays of light fall on a thin glass, part of the 

 rays being reflected from the upper surface, other rays, 

 reflected from the lower surface, might be so retarded 

 in their course through the glass that the two sets would 

 interfere with one another, the forward pulsation of one 

 ray corresponding to the backward pulsation of another, 

 thus quite neutralizing the effect. Some of the com- 

 ponent pulsations of the light being thus effaced by 

 mutual interference, the remaining rays would no longer 

 give the optical effect of white light; hence the puz- 

 zling colors. 



By following up this clew with mathematical preci- 

 sion, measuring the exact thickness of the plate and the 

 space between the different rings of color, Young was 

 able to show mathematically what must be the length 

 of pulsation for each of the different colors of the spec- 

 trum. He estimated that the undulations of red light, 

 at the extreme lower end of the visible spectrum, must 

 number about 37,640 to the inch, and pass any given 

 spot at a rate of 463 millions of millions of undulations 

 in a second, while the extreme violet numbers 59,750 

 undulations to the inch, or 735 millions of millions to 

 the second. 



Young similarly examined the colors that are pro- 

 duced by scratches on a smooth surface, in particular 

 testing the light from " Mr. Coventry's exquisite mi- 

 crometers," which consist of lines scratched on glass at 

 measured intervals. These microscopic tests brought 

 the same results as the other experiments. The colors 

 were produced at certain definite and measurable angles, 

 and the theory of interference of undulations explained 



197 



