NOTES. 



427 



of air corresponding to each color, from the breadth of the rings, which 

 are always of the same color with the homogeneous light. 



NOTE 196, p. 168. The focal length or distance 

 of a lens is the distance from its center to the point 

 F, fig. 60, in which the refracted rays meet. Let 

 L L' be a lens of very short focal distance fixed in 

 the window-shutter of a dark room. A sunbeam 

 S L L', passing through the lens, will be brought 

 to a focus in F, whence it will diverge in lines 

 PC, FD,and will form a circular image of light 

 on the opposite wall. Suppose a sheet of lead, 

 having a small pin-hole pierced through it, to be 

 placed in this beam ; when the pin-hole is viewed 

 from behind with a lens at E, it is surrounded with 

 a series of colored rings, which vary in appear- 

 ance with the relative positions of the pin-hole 

 and eye with regard to the point F. When the 

 hole is the 30th of an inch in diameter and at the 

 distance of 6A feet from F, when viewed at the 

 distance of 24 inches, there are seven rings of the 

 following colors : 



1st order: White, pale yellow, yellow, orange, 

 dull red. 



2d order : Violet, blue, whitish, greenish yellow, 

 fine yellow, orange red. 



3d order: Purple, indigo, blue, greenish blue, 

 brilliant green, yellow green, red. 



4th order : Good green, bluish white, red. 



5th order: Dull green, faint bluish white, faint 

 red. 



6ih order : Very faint green, very faint red. 



7th order : A trace of green and red. 



NOTI 197. p. 168. Let LL', fig. 61, 

 be the section of a lens placed in a 

 window-shutter, through which a very 

 small beam of light S L L' passes into 

 a dark room, and comes to a focus in F. 

 If the edge of a knife KN be held in 

 the beam, the rays bend away from it 

 in hyperbolic curves K r, K r', &c. in- 

 stead of coming directly to the screen 

 in the straight line K E", which is the 

 boundary of the shadow. As these 

 bending rays arr.ve at the screen indif- 

 ferent states of undulation, they inter- 

 fere, and form a series of colored fringes, 

 rrj. &.c. along the edge of the shadow 

 K E S X of the knife. The fringes vary 

 in breadth with the relative distances 

 of the knife edge and screen from F. 



