ON LIGHT AND COLOURS. 157 



breadth of the fringes by the different flexibility of the rays ; 

 and the reasoning in one of the foregoing propositions shows 

 how this inquiry may be conducted. But one thing is 

 certain, and probably Sir I. NEWTON had made the experi- 

 ment and grounded his opinion upon the result. If you 

 place a screen, with a narrow slit in the prismatic spectrum's 

 rays, parallel to the rectilinear sides, and then place a second 

 prism at right angles to the first and between the screen 

 and the chart, you will see the image of the slit drawn on 

 one side, the violet being furthest drawn, the red least 

 drawn ; but you will find no difference in the breadth of 

 the image cast by the slit. Flexion, however, operates in 

 a different manner, because it acts on rays, which, though 

 of the same flexibility, are at different distances from the 

 body. 



6. The internal fringes in the shadow (said by interference) 

 deserve to be examined much more minutely than they ever 

 have been ; and I have made many experiments on these, by 

 which an action of the rays on one another is, I think, 

 sufficiently proved. I shall here content myself with only 

 stating such results as bear on the question of interference 

 affecting my own other experiments. First. I observe that 

 when one side of a needle or pin is grooved so as to be partly 

 curvilinear, the other side remaining straight, we have in- 

 ternal fringes of the form in fig. 21. Secondly. It is not at 

 all necessary the pin or other body forming them should 

 be of very small diameter, although it is certain that the 

 breadth of the fringes is inversely as the diameter. I have 

 obtained them easily from a body one-quarter or one-third of 

 an inch in diameter, but they must be received at a con- 

 siderable distance from the body. Thirdly, and this is very 

 material as to interference at all affecting my experiments, 

 although certainly the internal fringes vanish when the rays 

 are stopped coming from the opposite side of the object, the 

 external fringes are not in the smallest degree affected, unless 

 you stop the light coming on their own side ; stopping the 

 opposite rays has no effect whatever. Thus, stopping the 



