Mr. Brougham’s Experiments , & c. 353 
trum was cast on a chart placed at right angles to the incident 
rays, and four feet from the prism. 
In the rays, parallel to the chart, and two feet from it, I 
placed a pin, whose diameter was ~ of an inch, and fixed it so, 
that the axis of its shadow on the spectrum might be parallel 
to the sides of the spectrum. A set of images by reflexion was 
formed (similar to those described above*), all inclining to the 
violet ; but what I chiefly attended to at present was their 
shape. I had always observed that the part formed out of the 
red-making rays was broadest, and that the other parts dimi- 
nished in breadth regularly towards the violet. I now deli- 
neated one or two, at about three inches from the shadow ; and 
though (from the pin’s irregularities) the sides were by no 
means smooth, yet the general shape was in every pin, and 
with every prism used, nearly as represented in fig. 1. (Tab.X.) 
divided in the direction R A, according to the colours of the 
spectrum in which they were formed ; R O B A was red, and 
the broadest ; that is, R A was broader than O B, the confines 
of the red and orange; and G D E V was the viplet, narrowest 
of all. 
Observation 2. Between the pin and the prism, ~ of an inch 
from the pin, was placed a screen, through a small hole in 
which, of twice the pin’s diameter, the rays of the spectrum pass- 
ed, and were reflected into images by the pin; these were pretty 
distinct and well defined, when received on a chart ± a foot from 
the pin. They were oblong, having parallel sides and confused 
ends ; they were wholly of the colour whose rays fell on the 
pin, unless when the white, mixed with those at the confines 
of the yellow and green, caused the images to be of all the 
* Phil. Trans, for 1796, page 24.0. 
