498 



NOTES. 



cession over the lenses, Newton also determined the thickness of the film of air 

 corresponding to each colour, from the breadth of the rings, which are always 

 of the same colour with the homogeneous light. 



NOTE 196, p. 194. The focal length or distance of 

 a lens is the distance from its centre to the point F, 

 fig. 60, in which the refracted rays meet. Let L I/ 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 FC, F D, 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 coloured 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 6$ 

 feet from F, when viewed at the distance of 24 inches, 

 there are seven rings of the following colours: 



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

 red. / 



2nd order : Violet, blue, whitish, greenish yellow, fine Q 

 yellow, orange red. 



3rd order: Purple, indigo, blue, greenish blue, bril- 

 liant green, yellow green, red. 



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



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



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



7th order : A trace of green and red. 



NOTE 197, ? 195. 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 

 Kr, K/, &c., instead of coming directly to 

 the screen in the straight line KE, which 

 is the boundary of the shadow. As these 

 bending rays arrive at the screen in different 

 states of undulation, they interfere, and 

 form a series of coloured fringes, rr', &c., 

 along the edge of the shadow KESN of 

 the knife. The fringes vary in breadth with 

 the relative distances of the knife-edge and 

 screen from F, 



