RELATIVE TO PHYSICAL OPTICS. 



641 



at a very acute angle^ in a beam of the sun's 

 light, admitted through a small aperture; 

 and the point of concourse of the two first 

 dark lines, bordering the shadows of the re- 

 spective knives, was observed at various dis- 

 tances. The results of six observations are 

 expressed in the first three lines of the first 

 Table. On the supposition that the dark line 

 is produced by the first interference of the 

 light reflected from the edges of the knives, 

 with the light passing in a straight line be- 

 tween them, we may assign, by calculating 

 the difference of the two paths, the interval for 



the first disappearance of the brightest light, 

 as it is expressed in the fourth line. The se- 

 cond Table contains the results of a simi- 

 lar calculation, from Newton's observations 

 on the shadow of a hair ; and the third, 

 from some experiments of my own, of the 

 same nature ; the second bright line being 

 supposed to correspond to a double interval, 

 the second dark line to a triple interval, 

 and the succeeding lines to depend on a con- 

 tinuation of the progression. The unit of all 

 the Tables is an inch. 



Table I. Obi. g. N. 



Distance of the knives from the aperture 

 Distances of the paper from the 



knives . . \\, 3\, 8 



Distances between the edges of 



the knives, opposite to the 



TJ 



point of concourse . 

 Interval of disappearance 



.012, 



.020, 



.034, 



32, 



.057, 



96, 



.081 



JOh 



131. 



.087. 



.0000122, .0000155, .0000182, .00001G7, .0000166, .0000166. 

 Table II. Obs. 3. N. 



Breadth of the hair . . ..... 



Distance of the hair from the aperture 



Distancesof the scale from the aperture 



(Breadths of the shadow * . 



Breadth between the second pair of bright lines 



Interval of disappearance, or half the difference of the paths 



Breadth between the third pair of bright lines 



Interval of disappearance, i of the difference 



Table III. Eiper. 5. 



Breadth of the object . . . . ; 



Distance of the object fiorn the aperture 



Distance of the wall from the aperture 



Distance of the second pair of darkUnjes from each other 



Interval of disappearance, 4- of the difference . , 



150, 



T4» 



.0000151, 



4 

 7T» 



.0000150, 



2. To" 



144 



252. 



y) 



♦ 



.0000173. 

 .0000143. 



.4r,4. 



125. 



250. 



1.1 67. 



.0000149. 



vol. If. 



4 o 



