23.2 Rorjal Society. 



Though the phenomenon seem to indicate a crossing of the rays 

 both in flexion and reflexion, at or near the distance at which the 

 dark or deeji purple line is formed, ytt the author has never been 

 able to observe that an obstacle placed between that point and the 

 speculum (or the bending edges), made the fringes on the opposite 

 side of the disc at the screen to disappear, but only the fringes on 

 the same side with itself. 



Referring to Fresnel's memoir, the author states that the princijjle 

 laid down in it, " that the dilatation of the fringes depends solely upon 

 the breadth of the aperture," will not afford an explanation of the 

 phenomena described in his former paper respecting fringes formed 

 by edges acting in succession, for he there showed that their breadth 

 and their distances from the direct rays are in the inverse proportion 

 of the distance of the edges ; and if the edges are so placed that the 

 rays pass parallel to each other, and not diverging, and the edges are 

 moved to diff'erent distances in the same line, e. g. horizontally, then 

 their distance from each other vertically being the same, the aperture 

 is the same at all distances of the edges from each other horizontally, 

 and yet the breadth of the fringes is inversely as the horizontal di- 

 stance. Further, where the edges are not placed in succession, but 

 directly opposite to each other, the breadths of the fringes do not 

 appear to follow the exact inverse proportion of the distances of the 

 edges (that is the size of the aperture), the observed breadths corre- 

 sponding more nearly with the curve y = — I — . ^ being the distance 



XX- 



of the edges, and y the breadth of the fringes. 



The author considers that the internal fringes, or those of the 

 shadows of small bodies, called fringes of interference, require a more 

 full examination than they have received in certain respects. As re- 

 gards the central space and the two deep black fringes or intervals 

 on each side of it, he remarks that no examination with a magnifier, 

 and no inclination of the screen, at all resolves these colours into 

 purple as in the dark line before described. They appear to follow 

 a diff'erent law from that of the coloured ones as regards their breadths 

 in proportion to their distances from the pin or other small object, 

 at least if they are caused by interference, and if the eff'ect of inter- 

 ference is inversely as the difference of the length of the rays ; for 



m 

 that would give for the breadths the curve v= / — =^ . . 



which nowise agrees with the admeasurements. 



The action of transparent plates on the rays, in bending them, re- 

 sembles in every respect that of opake jilates, except that there being 

 no shadow, the external fringes are not perceived. But the shadow 

 of the edge of the plate is surrounded by two sets of fringes resem- 

 bling exactly those surrounding the shadow of a hair or other small 

 body placed upon the plate's edge, and following its course, with 

 this only diff'crence, that this shadow of the transparent plate's edge 

 has no internal fringes as the hair or other small body's shadow has. 



. May 6. — A paper was read, entitled, " On Periodical Laws disco- 

 verable in the mean effects of the larger Magnetic Disturbances." — 

 No. II. By Colonel Edward Sabine, R.A., Treas. and V.P.R.S. &c. 



