354 Mr. Brougham's Experiments and Observations 
colours. When the prism was turned round on its axis, so 
that different rays fell on the pin, the images changed their 
sizes as well as their positions ; they were largest when red, and 
least when violet. 
Observation 3. In case it may be thought that the sides of 
the hole, through which the rays passed in observation 2, by 
inflecting, might dispose them, before incidence, into beams of 
different sizes, I removed the screen, and placed the pin hori- 
zontally, the axis of the shadow being now at right angles to 
that of the prismatic spectrum; and moving the prism on its 
axis, again I observed the contraction and dilatation of the 
images by reflexion, though now they were rather less dis- 
tinct, from the greater size of the incident beam ; and to shew 
that there was both a change of size and of place, without any 
manner of deception, I placed one leg of a pair of compasses 
in a fixed point of the spectrum, and the other in the middle 
point of an image formed by the violet-making rays. The 
prism being then moved till the image became red, I again bi- 
sected it, and found its centre considerably beyond the point 
of the compasses, which was indeed evidently much nearer one 
end of the image than the other ; besides, that the red image, 
when measured, was longer than the rest; and this satisfied 
me that there were two changes, one of place, with respect to 
the fixed point, the other of size, with respect to the centre of 
the image. Lastly, as far as I could judge, the dilatation and 
contraction appeared even and uniform. 
Observation 4. I remarked that the fringes or images, by 
flexion, were always increased in size when formed out of red- 
making rays, and were less in every other colour, and least in 
violet (besides being moved farther from the edge of the shadow 
