A HISTORY OF SCIENCE 



breadth and a half, again destroyed; and thus for a 

 considerable number of alternations, and if the re- 

 flected undulations be of a different kind, they will be 

 variously affected, according to their proportions to 

 the various length of the line which is the difference 

 between the lengths of their two paths, and which may 

 be denominated the interval of a retardation. 



" In order that the effect may be the more percepti- 

 ble, a number of pairs of points must be united into 

 two parallel lines ; and if several such pairs of lines be 

 placed near each other, they will facilitate the ob- 

 servation. If one of the lines be made to revolve 

 round the other as an axis, the depression below the 

 given plane will be as the sine of the inclination; and 

 while the eye and the luminous object remain fixed 

 the difference of the length of the paths will vary as 

 this sine. 



"The best subjects for the experiment are Mr. Cov- 

 entry's exquisite micrometers; such of them as con- 

 sist of parallel lines drawn on glass, at a distance of one- 

 five-hundredth of an inch, are the most convenient. 

 Each of these lines appears under a microscope to con- 

 sist of two or more finer lines, exactly parallel, and at a 

 distance of somewhat more than a twentieth more than 

 the adjacent lines. I placed one of these so as to re- 

 flect the sun's light at an angle of forty-five degrees, 

 and fixed it in such a manner that while it revolved 

 round one of the lines as an axis, I could measure its 

 angular motion; I found that the longest red color 

 occurred at the inclination ioj, 20^, 32, and 45; of 

 which the sines are as the numbers i, 2, 3, and 4. At 

 all other angles also, when the sun's light was reflected 



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