172 Dr. Herschei/s Experiments for Investigating 
that a similar power will be exerted by all sorts of curvatures 
and irregularities of reflecting surfaces ; but in order to take 
nothing upon trust that may be proved, I had recourse to the 
following experiments with different curvatures. 
Second set of experiments. Upon a ridge of glass ground 
and polished to a cylindrical form, I laid the base of a prism, 
with one angle of 96 ; and two of 42 degrees each. In this 
I saw beautifully coloured streaks, or rather very narrow 
lenticular configurations, on one side as low as the angle of 
42 0 , will allow the critical separation to be seen, and on the 
other down to within about 5 0 of the level of the plane, which 
gives a range of visibility of nearly 82 degrees. 
Having also provided a refracting angle of 3 0 43' 8", and 
Iain it upon the same cylindrical ridge, the visibility of these 
phenomena was extended to 152 degrees. 
A plain glass laid upon the same ridge, extended this range 
to 170 degrees. 
That these effects of extending the range of the angular 
space, in which the narrow coloured lenticular forms are 
visible, is owing to the modifying power of the cylindrical 
surface, is particularly evident from the parallelism of the 
coloured phenomena with the line of contact, and from the 
direction of their extended visibility at right angles to this 
line. 
Having laid a right angled prism upon the same ridge, I 
perceived not only the coloured primary figure, but also a 
similar, magnified secondary one, which I ascribed to a re- 
flection from the flat base of the glass, the upper side of 
which, contained the polished cylindrical ridge. To take off 
this secondary image, I deprived the base of its reflecting 
