468 PHYSICAL SCIENCES. 



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



6th ring : Pale blue green, pale pink. 



7th ring: Very pale blueish 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. 



NOTE 200, p. 172. The absolute thickness of the film of air between 

 the glasses is found as follows: Let AFBC, fig. 59, be the section of a 

 lens lying on a plane surface or plate of 

 y - glass P P', seen edgewise, and let E C be 



the diameter of the sphere of which the 

 lens is a segment. If A B be the dia- 

 meter of any one of Newton's rings, and 

 B D parallel to C E, then B D or C F is 

 the thickness of the air producing it. 

 E C is a known quantity ; and when A B, 

 the diameter, is measured with compasses, 

 BDor FC can be computed. Newton 

 found that the length of B D, correspond- 

 ing to the darkest part of the first ring, 

 is the 98,000th part of an inch when 

 the rays fall pei*pendicularly on the lens, 

 and from this he deduced the thick- 

 ness corresponding to each colour in the system of rings. By passing 

 each colour of the solar spectrum in succession 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 201, p. 174. 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 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 

 F C, 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 

 appearance 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 red. 



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

 red. 



3rd order: Purple, indigo blue, greenish blue, brilliant green, yellow 

 green, red. 



4th order: Blueish green, blueish white, red. 



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



