ASTRONOMY, ETC., OF SEVENTEENTH CENT. 223 



was acquainted, to measure their thickness. Newton took up the in- 

 vestigation, and, with his usual sagacity, overcame the difficulty of 

 measuring these extremely small spaces. He formed a film of air by 

 pressing together two lenses of very large but known curvatures. The 

 curvature of the convex lens was somewhat greater than that of the con- 

 cave surface to which it was applied. Fig. 107 represents the lenses 



FIG. 108. 



in section. As the radius of each curved surface is known, it is easy 



to calculate the distances which separate the glasses at any given dis- 



tance from the central point e, where they are in contact. The double 



curvature is not essential to the success of the experiment, and the 



reader will be able readily to make the experiment of " Newton's Rings" 



by pressing an eye-glass or spectacle-lens of small curvature against a 



Hat piece of plate glass. Fig. 108 may represent the section of the 



glasses in this case, and an obvious method of calculating the thick- 



ness of the separating film at any given distance, as fg from the point 



of contact, will probably suggest itself even to an ungeometrical reader, 



who will apply the famous theorem, mentioned on page 13, to the 



triangle cgf. The colours, as seen in the admirable experiment of 



Newton, present themselves in concentric rings, which, by ordinary 



light, appear of various colours ; which, however, are not in the order 



of the colours of the prismatic spectrum. When only one kind of light 



is admitted as, for instance, when it is first passed through red glass 



a series of alternate light and dark rings are seen, as represented in 



Fig. 109 on a large scale. If yellow light that, for instance, which is 



given off from a spirit-lamp with a salted wick be employed, the dark 



and light rings will be more contracted than those formed with red light, 



blue light will give rings of more contracted dimensions than yellow 



light, and so on. If the several series of rings which each coloured 



light is capable of producing were superimposed, the effect would be 



that observed when the film is viewed in ordinary light. By examin- 



