the Inflection, Reflection, and Colours of Light. 231 
by black cloth hung before the window, and in the room, at 
the hole I placed a prism of glass, whose refracting angle was 
45 degrees, and which was cc-vered all over with black paper, 
except a small part on each side, which was free from impu- 
rities, and through which the light was refracted, so as to form 
a distinct and tolerably homogeneous spectrum on a chart at 
six feet from the window. In the rays, at two feet from the 
prism, I placed a black unpolished pin (whose diameter was 
every where one-tenth of an inch) parallel to the chart, and 
in a vertical position. Its shadow was formed in the spectrum 
on the chart, and had a considerable penumbra, especially in 
the brightest red, for it was by no means of the same thick- 
ness in all its parts ; that in violet was broadest and most 
distinct; that in the red narrowest and most confused, and 
that in the intermediate colours was of an intermediate thick- 
ness and degree of distinctness. It was not bounded by 
straight, but by curvilinear sides, convex towards the axis to 
which they approached as to an asymptote, and that, nearest 
in the least refrangible rays, as is represented in fig. 2. where 
AB is the axis, IKLMNA and HGFEDA the two outlines. 
Nor could this be owing to any irregularity in the pin, for the 
same thing happened in all sorts of bodies that were used ; and 
also if the prism was moved on its axis, so that the colours 
might ascend and descend on these bodies, still wherever the 
red fell it made the least, and the violet the greatest shadow. 
Obs. 2. In the place of the pin, I fixed a screen, having in 
it a large hole on which was a brass plate, pierced with a small 
hole^-of an inch in diameter ; then causing an assistant to move 
the prism slowly on its axis, I observed the round image made 
by the different rays passing through the hole to the chart; that 
