206 PHILOSOPHICAL TRANSACTIONS. [ANNO 1797. 



nermost rings made by plates have their diameters (not breadths) in the ratio of 

 1-14- to 2-§-*, and the distance between the middle of the innermost fringes, made 

 by a hair, on either side the shadow, is to the same distance in the 2d fringes, as 

 ^i_ to ^-; therefore the diameters of the first 2 rings made by the specks in the 

 speculum, are as -§-§-§- to -rinrri which ratio differs exceedingly little from that of 

 1-ff to 2f, the ratio of the diameters of rings made by plates, either those called 

 by Newton thick, or those which he names thin: for suppose this difference 

 nothing, 2-f- X -H4 = ] t4- X -rVsV; an d tne difference between these 2 products, 

 now stated equal, is not much above -§- in reality. 



Observ. 8. The last thing worth mentioning in these phenomena was this: I 

 viewed the fringes through a prism, holding the refracting angle upwards, and the 

 axes parallel to that of the dark space; then moving it till the objects ceased 

 descending, I saw in that posture the fringes much more distinct and numerous; 

 for I could now see 5 with ease, and several more less distinctly. This led me to 

 try more minutely the truth of the 5th proposition, with respect to the number of 

 the fringes surrounding the shadows of bodies in direct light. Having produced a 

 bright set of these by a blackened pin -^ of an inch in diameter, I viewed them 

 through a well made prism, whose refracting angle was only 30°, and held this 

 angle upwards, when the fringes were on the side of the shadow opposite to me; 



1 then moved the prism round on its axis, and when it was in the posture between 

 the ascent and descent of the objects, I was much pleased to see 5 fringes plainly, 

 and a great number beyond, decreasing in size and brightness till they became too 

 small and confused for sight. In like manner those formed by a double flexion of 



2 bodies, and those made out of homogeneal light, were seen to a much greater 

 number when carefully viewed through the prism. And this experiment I also 

 tried with all the species of fringes by flexion which I could think of. 



Observ. Q. The same appearances which were occasioned by the metal speculum, 

 might be naturally expected to appear when a glass one was used. But I also found 

 the like rings or fringes of colours and spots in the image beyond the focus of a 

 lens; nor was a very excellent one belonging to a Dollond's telescope free from them. 

 The rings with their dark intervals resembled those floating specks so often ob- 

 served on the surface of the eye, and called " muscae volitantes," only that the 

 muscae are transparent in the middle, because formed hydrops of humor: they will 

 however be found to be compassed by rings of faint colours, which will become 

 exceedingly vivid if the eyes be shut and slowly opened in the sun's light, so that 

 the humour may be collected; they also appear by reflexion, mixed with the colours 

 described in Phil. Trans, for 1796. 



Observ. 10. The sun shining strongly on the concave metal speculum, placed at 

 such a distance from the hole in the window that it was wholly covered with the 

 light; on inclining it a little, the image on the chart was bordered on the inside 

 with 3 fringes similar to those already described; on increasing the inclination these 



* Book 2. Parts 1 and 4. — Orig. 



