313 



tiot remarked. These experiments are either made with two bodies 

 acting jointly on the rays, or with a single body so acting. 



3. When made with two bodies, as sharp edges, these edges 

 must be perfectly parallel, and when placed in the axis of the pris- 

 matic spectrum they form fringes more distant in the red than in 

 any other part ; least distant in the violet. The fringes are both 

 broadest in the least refrangible rays and most removed ; narrowest 

 and least removed in the most refrangible. They incline from the 

 red towards the violet. 



4. The same experiment is easily made with a lamp or candle, 

 placing a prism between the flame and the edges. This renders 

 that exact parallelism of the edges which is required in the experi- 

 ment with the spectrum, comparatively immaterial ; because a con- 

 siderable inclination of the edges, as at an angle of half a degree or 

 more, does not affect the action on the rays in the very small space 

 through which they pass by the edges. 



5. With a single edge, or other body as a hair, the same difference 

 in the position, as well as in the breadth of the fringes, is found to 

 be observable, though not so manifestly as when two act together on 

 the light; The manner of making the observation most conveniently 

 is pointed out. 



6. These experiments are varied so as to show the various disten- 

 sions of the disc of a flame subjected to flexion between two edges, 

 according as we regard the various portions of the flame's spectrum 

 when seen by the prism. The red part is broadest, and when the 

 near approach of the edges to each other divides the disc into two 

 with a dark interval between them, that interval is the broadest in 

 the least refrangible rays. 



7. The experiments are further varied by using coloured glass 

 instead of refracting with a prism. 



8. The same phenomena are found to exist in all the other cases 

 of flexion as where it is combined with reflexion by the action of 

 specula, or by the effect of striated surfaces. There is always the 

 same difference in the effects produced by the different kinds of 

 homogeneous light. 



9. The same phenomena are not so easily observed in the internal 

 fringes, or those of the shadow ; but the dark gray line in the axis 

 of the shadow, having a line of deep black on each side, is found to 

 vary in breadth and position in the different parts of its length cor- 

 responding to the colours of the spectrum, v/hen a needle or other 

 small body is placed in that spectrum. 



10. The angle of inflexion is shown to be obtained by taking the 

 distance at which the internal fringes begin to appear ; and the pro- 

 portion of this angle in the red to the same angle in the violet is 

 ascertained. The deflexion (as deduced from Sir I. Newton's ex- 

 periments) is much greater than inflexion appears to be. He had 

 not observed the internal fringes of Grimaldi, to whom, however, he 

 refers. 



11. The author states that the property in question, the different 

 flexibility of light, coexists with the other property, whatever it may 



