VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 205 



a prism ; this being turned on its axis, the rays which fell on the speculum were 

 changed ; the fringes were now of that colour whose rays fell, and when the rays 

 shifted, the fringes contracted or dilated, being broadest in the most flexible rays, 

 and consequently in those whose flexity is greatest. 



Observ. 5. The direct light falling on the speculum, and part of the reflected 

 light on the horizontal white stage of a very accurate micrometer, I measured the 

 breadth of the fringes, spots, &c. These, with the distance of the speculum from 

 the window and micrometer, and the size of the sun's image, are set down in the 

 following table, all reduced to inches and decimals. 



Inches. Parts. 



Distance of the speculum from the hole in the Breadth of the oblong dark spot 0074 



window shutter 24 Breadth of its first fringe 0022 



Distance of the speculum from the stage of the Elliptic spot's transverse axis 01 16 



micrometer 18 conjugate axis 0068 



Transverse axis of the sun's image 2.6 Breadth of its first fringe 0034 



Conjugate axis of the sun's image 1.4 Transverse axis of a larger elliptic spot . . . .013 



Length of the oblong dark spot 4 Conjugate axis of the same spot 0076" 



In the image where these measures were taken, there were 7 other elliptic spots, a 

 little less and nearly equal; all the others were much smaller and more confused. 



Observ. 6. On viewing the surface of the speculum attentively in that place 

 whence the rays formed the oblong and first mentioned elliptic spots, I saw a dark 

 but very thin long scratch, and a dark dent, similar in shape to the dark spaces on 

 the image; the dark spot measured less than -,-i-g- of an inch; which makes its 

 whole surface to the whole polished surface as 1 to 34225, supposing the former 

 circular or nearly so. All these measures will be found to agree very well, for their 

 smallness and delicacy : thus, the ratio last mentioned is nearly the same which we 

 obtain by comparing the image and the spot ; the like may be said of the two spots 

 mentioned in the table, i. e. their axes are proportional. I now could produce what 

 spots I pleased, by gently scratching the speculum, or by making lines, dots, &c. 

 with ink, and allowing it to dry ; for these last formed convex fibres, which pro- 

 duced coloured fringes as well as the concavities, agreeably to what was deduced a 

 priori. 



Observ. 7. The whole appearance which I have been describing bore such a close 

 and complete resemblance to the fringes made round the shadows of bodies, that 

 the identity of the cause in both cases could not be doubted. In order however to 

 show it still further, I measured the breadths of 2 contiguous fringes in several 

 different sets; the measurements agreed very well, and gave the breadth of the 

 first fringe .005(3, and of the 2d .0034; or of the first .0066, and of the 2d .0034. 

 The ratio of the breadths by the first is 28 to 17; by the 2d 30 to 17; of which 

 the medium is 29 to 17, and this is precisely the ratio of the 2 innermost fringes 

 made by a hair, according to Sir Isaac Newton's measurement: the first being, 

 according to him, ^fy °f an mcn 5 tne 2c * yH °f an inch*. Further, the 2 in- 



* Optics, Book 3. Obs. 3.— Orig. 



