the Cause of Coloured Concentric Rmgs. 1 73 
power, by rubbing it on emery, which had the required 
effect. 
A doubt might occur, with regard to the cause which pro- 
duced the colours, because in these experiments the subjacent 
medium was of glass ; and although on account of the 
emeried base, we may be certain that the curved ridge only 
acted by reflecting the critically separated transmitted colours 
in a variety of angles, yet to prove in the most satisfactory 
manner, that these colours were exclusively from the prism, 
I ground and polished two metalline cylindrical pieces to 
different curvatures, and laying upon them my small pris- 
matic angle of 3 0 43', the minute lenticular figures, became 
again visible over an extent of 152 degrees. 
To vary the experiment, I placed a cylindrical ridge in 
contact with a plain metalline mirror, and saw with great 
facility through the plain side of the glass which was towards 
the eye, and was highly polished, the extended range of the 
modified colours, which in this case amounted to 170 degrees. 
From this variation a very important consequence may be 
drawn, which is, that a radiation from curved surfaces re- 
flected by plain ones, will effect the same extension of visi- 
bility, as a radiation from plain surfaces reflected by curved 
ones. How much more then must this effect be produced, 
when two curved surfaces are applied to each, as for instance, 
when two double convex object glasses are laid together ? 
Third set of experiments. Upon a piece of mica tied over 
a cylinder, I placed a right angled prism, and having brought 
the eye to the altitude where critically separated rays are 
visible, I perceived a number of irregular forms of beautiful 
colours. When the position of the eye was gradually changed, 
mdcccx* A a 
