on the Affections and Properties of Light. 371 
he names thin : for suppose this difference nothing, 2-| x •§-§-§- == 
i~ x Y5V 3 * an d the difference between these two products (now 
stated equal) is not much above in reality. 
Observation 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 axis parallel to 
that of the dark space ; then moving it till the objects ceased 
descending, I saw in that posture the fringes much more dis- 
tinct and numerous ; for I could now see five 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 awell made prism, 
whose refracting angle was only 30°, and held this angle up- 
wards, when the fringes were on the side of the shadow oppo- 
site to me; I then moved the prism round on its axis, and when 
it was in the posture between the ascent and descent of the ob- 
jects, I was much pleased to see five fringes plainly, and a 
great number beyond, decreasing in size and brightness till 
they became too small and confused for sight. In like man- 
ner those formed by a double flexion of two 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. 
Observation 9. 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 
MDCCXCVII. 3 C 
