ON LIGHT AND COLOUKS. 151 



of a b. If the second edge is furnished with a similar curve 

 surface the form is more complete, as in c d. But the straight 



Eg.18. 



edge being used after the first flexion of the curved one, 

 clearly shows that the first edge bends as well as the second, 

 indeed more than the second, for the side of the figure 

 answering to that curved edge is most curved. Fourthly, the 

 whole experiments with two edges directly opposite each 

 other negative the idea of there being no inflexion ; indeed 

 they seem to prove the inflexion equal to the deflexion. The 

 phenomena under Proposition X. can in no way be re- 

 conciled to the supposition of the first edge not inflecting 

 the rays.* 



2. \Ve must ever keep in view the difference between the 

 fringes or images described by Sir I. NEWTON and measured 

 by him, as made by the rays passing on each side of a hair, 

 and the fringes or images which are made without the inter- 

 ference of rays passing on both sides. It is clear that the 

 rays which form those fringes with their dark intervals do 

 not proceed after passing the hair in straight lines. Sir I. 

 N EWTON'S measures | prove this ; for at half a foot from the 

 hair he found the first fringe T ^g-th of an inch broad, and the 

 second fringe ^-^ ; and at nine feet distance the former were 

 ^'j, the latter 3*5-, instead of between -i- and T V, and the latter 

 less than T ^, and so of all the other measures in the table, 

 each being invariably about one-third what it ought to be if 

 the rays moved in straight lines ; and this also explains why 

 the fringes do not run into one another, or encroach on the 



* If you hold a body between the eye and a light, as that of a candle, 

 and approach it to the rays, you see the flame drawn towards the body ; 

 and a beginning of images or fringes is perceived on that side. 



t Optics, B. iii. obs. 3. 



