GO 



A POPULAR ACCOUNT 



20, or 31 to 10, or as 310,000,000, 

 100,000,000, which numbers are very 

 nearly as the squares of 17,607 and 

 10,000, and in the proportion of the first 

 of these to the second, are the diameters 

 of the bright rings made by the thinner 

 glass, 3, 4i, 5, to the diameters of the 

 corresponding rings made by the thicker 

 glass, l{i, 2f, 2i|. This perfect^accord- 

 ance of the phenomena with the results 

 of theory was considered by N ewton to be 

 conclusive as to the validity of his hypo- 

 thesis. 



CHAPTER IX. 



Experiments on the Inflexion of Light. 

 (83.) IN the account of the state 

 of optical science before the time of 

 Newton, which was given in the first 

 chapter, the discovery of the diffraction 

 or inflexion of light by Grimaldi was 

 noticed. This phenomenon was too 

 striking to escape the attention of 

 Newton. He accordingly instituted some 

 experiments with a view to the investi- 



gation of this property ; but he seems not 

 to have had leisure or inclination to pur- 

 sue the subject to as great an extent, or 

 to obtain results so satisfactory as those 

 which have been discussed in the pre- 

 ceding chapters. In the commencement 

 of his third book, however, be describes 

 some experiments which he made, and 

 hazards some conjectures on the pro- 

 bable cause of these phenomena, which, 

 as well as some other matters connected 

 with physical science in general, and 

 with the phenomena of heat and light in 

 particular, he proposes in the form of 

 queries. He states that he was inter- 

 rupted in his investigations; that he could 

 not afterwards think of taking these 

 things into further consideration ; and as 

 he had not finished his design, he leaves 

 these queries, in order to a further search 

 to be made by others. 



The experiments which led Grimaldi 

 to the discovery of inflexion were the 

 following. Through a small aperture 

 A B (fig. 51) he admitted a beam of 



M I G K L 



light into a dark room. He observed 

 that the light thus admitted, spread itself 

 in the form of a cone, and illuminated a 

 large portion of a screen C D, held per- 

 pendicular to its direction, and at some 

 distance from the hole, the illuminated 

 part of this screen increasing with the in- 

 crease of distance. When an opaque 

 body E F was held in the light so as to 

 cast a shadow on the screen C D, this 

 shadow was found to be much larger 

 than it would have been if the light had 

 passed the edges of E F in right lines. 

 From B draw the straight line B F G 

 and from A the line A E H. Also from 

 A draw the right line A F I, and from B 

 the right line BEL. Now, if the light 

 passes the edges of E F in right lines, 

 it. is plain that the space G H will be 

 altogether deprived of light, and that 

 from the limits G H to I and L respec- 

 tively there will be a partial orpenumbral 

 illumination ; but that beyond the limits 

 I L,the illumination of the screen will be 

 as complete as if the opaque object E F 

 were not present. This was, however, 



00, IV 



]) 



found not to be the fact, for the shadow 



extended beyond I and L to wider limits, 



as M and N. 



Beyond this shadow. Fig. 52. 



and skirting it, Grimaldi 



observed three coloured 



fringes, the broadest and 



brightest of which was 



next the shadow. If X 



(Jig. 52) represent the 



edge of the shadow, the 



space N N was blue, M 



was colourless, and O O 



red. In the second fringe, " JN" oq, RT V 

 Q Q was faint blue, P colourless, and 

 11 R red. This fringe was narrower than 

 the first and more faint. The third fringe 

 T T, S and V V, was similar to the other 

 two, but still narrower and fainter. 



"When the opaque body has a salient 

 angle as D (fig. 53), the fringes were 

 arched round it, and at a re-entrant 

 angle they intersected each other as at C. 

 Newton repeated and varied these ex- 

 periments of Grimaldi. He pierced a 

 plate of lead with a pin, making a hole 



