426 



NOTES. 



Fig. 58. 



a screen S S' at a little distance. When either one or other of the slits 

 O or O' is stopped, the dark bands vanish, and the screen is illuminated 

 by a uniform light, proving that the dark bands are produced by the in- 

 terference of the two sets of rays. Again, let H m, fig. 57, be a beam of 

 white light passing through a hold at H, made with a fine needle in a 

 piece of lead or a card, and received on a screen S S'. When a hair, or 

 a small slip of card hh' about the 30th of an inch in breadth, is held in 

 the beam, the rays bend round on each side of it, and, arriving at the 

 screen in different states of vibration, interfere and form a series of co- 

 lored fringes on each side of a central white band m. When a piece of 

 card is interposed at C, so as to intercept the light which passes on one 

 side of the hair, the colored fringes vanish. When homogeneous light 

 >s used, the fringes are broadest in red, and become narrower for each 

 color of the spectrum progressively to the violet, which gives the nar- 

 rowest and most crowded fringes. These very elegant experiments are 

 due to Df. Thomas Young. 



NOTE 194, pp. 165, 191. Fig. 58 shows Newton's rings, of which there 

 are seven, formed by screwing two lenses of 

 glass together. Provided the incident light be 

 white, they always succeed each other in the 

 following order: 



1st ring, or first order of colors : Black, very 

 faint blue, brilliant white, yellow, orange, red. 



2d ring: Dark purple, or rather violet, blue, 

 a very imperfect yellow green, vivid yellow, 

 crimson red. 



3d ring : Purple, blue, rich grass green, fine 

 yellow, pink, crimson. 



4th ring : Dull bluish green, pale yellowish pink, red. 



5th ring: Pale bluish green, white, pink. 



6th ring : Pale blue-green, pale pink. 



7th ring : Very pale bluish green, very pale pink. 



After the seventh order, the colors become too faint to be distinguished. 

 The rings decrease in breadth, and the colors become more crowded to- 

 gether, as they recede from the center. When the light is homogeneous, 

 the rings are broadest in the red, and decrease in breadth with every 

 successive color of the spectrum to the violet. 



NOTE 195, p. 166. The absolute 

 thickness of the film of air between 

 the glasses is found as follows : Let 

 A F B C, fig. 59, be the section of a 

 lens lying on a plane surface or plate 

 of glass PP', seen edgewise, and let 

 E C be the diameter of the sphere of 

 which the lens is a segment. If A B 

 be the diameter of any one of Newton's 

 rings, and B D parallel to C E, then B 

 D or CF is the thickness of the air 

 producing it. E C is a known quanti- 

 ty, and when AB the diameter is 

 measured with compasses, B D or F C 

 can be computed. Newton found that 

 the length of B D corresponding to the 

 darkest part of the first ring, is the 

 98,000th part of an inch when the rays fall perpendicularly on the lens, 

 and from this he deduced the thickness corresponding to each color in the 

 system of rings. By passing each color of the solar spectrum in succes- 

 sion over the lenses, Newton also determined the thickness of the film 



