NOTES. 



497 



is stopped, the dark bands vanish, and the screen is illuminated by a uniform 

 light, proving that the dark bands are produced by the interference of the two 

 sets of rays. Again, let H m, fig. 57, be a beam of white light passing through 

 a hole 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 h h', 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 coloured fringes on each side of a central white band m. When a 

 piece of card is hiterposed at C, so as to intercept the light which passes on 

 one side of the hair, the coloured fringes vanish. When homogeneous light is 

 used, the fringes are broadest in red, and become narrower for each colour of 

 the spectrum progressively to the violet, which gives the narrowest and most 

 crowded fringes. These very elegant experiments are due to Dr.Thomas Young. 



NOTE 194, pp. 191, 221. Fig. 58 shows Newton's rings, of which there are 

 seven, formed by screwing two lenses of glass to- ' Fig. 58. 



gether. Provided the incident light be white, they 

 always succeed each other in the following order : 



1st ring, or 1st order of colours: Black, very 

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



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

 a very imperfect yellow green, vivid yellow, crim- 

 son red. 



3rd 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 colours become too faint to be distinguished. 

 The rings decrease in breadth, and the colours become more crowded together, 

 as they recede from the centre. When the light is homogeneous, the rings are 

 broadest in the red, and decrease in breadth with every successive colour of the 

 spectrum to the violet. 



Fig. 59. 



NOTE 195, p. 193. The absolute thick- 

 ness 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 P P', seen edge- 

 wise, and let EC be the diameter of the 

 sphere of which the lens is a segment. If 

 A B be the diameter of any one of New- 

 ton's rings, and B D parallel to C E, then 

 B D or C F is the thickness of the air pro- 

 ducing it. E C is a known quantity, and 

 when A B, the diameter, is measured with 

 compasses, B D or F C can be computed. 

 Newton found that the length of BD p 

 corresponding to the darkest part of the 

 first ring, is the 98000th part of an inch when the rays fall perpendicularly on 

 the lens, and from this he deduced the thickness corresponding to each colour 

 in the system of rings. By passing each colour of the solar spectrum in suc- 



K E 



