A HISTORY OF SCIENCE 



beam and of the scattered light between the two sur- 

 faces; of course, wherever the inclination of the scat- 

 tered light is equal to that of the beam, although in 

 different planes, the interval will vanish and all the 

 undulations will conspire. At other inclinations, the 

 interval will be the difference of the secants from the 

 secant of the inclination, or angle of refraction of the 

 principal beam. From these causes, all the colors of 

 concave mirrors observed by Newton and others are 

 necessary consequences ; and it appears that their pro- 

 duction, though somewhat similar, is by no means as 

 Newton imagined, identical with the production of 

 thin plates." 2 



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 

 spectrum. He estimated that the undulations of red 

 light, at the extreme lower end of the visible spectrum, 

 must number about thirty-seven thousand six hundred 

 and forty to the inch, and pass any given spot at a rate 

 of four hundred and sixty-three millions of millions of 

 undulations in a second, while the extreme violet num- 

 bers fifty-nine thousand seven hundred and fifty un- 

 dulations to the inch, or seven hundred and thirty-five 

 millions of millions to the second. 



The Colors of Striated Surfaces 



Young similarly examined the colors that are pro- 

 duced by scratches on a smooth surface, in particular 



220 



