262 Dr. HerscheFs Experiments for investigating 
were to be seen, exceeded all the imaginary forms which the 
most inventive fancy can paint. The flexibility of mica also 
gave room for using different degrees of pressure, by which 
means a continual change of figure and succession of prismatic 
colours was produced. 
When I laid a piece of this mica upon a cylinder, and placed 
a plain slip of glass or double convex lens upon it, all its ir- 
regularities were modified into disfigured streaks with the 
former, and distorted ellipses with the latter. 
Experiments of a similar nature were made upon the irre- 
gular surface of Island crystal and other substances, which all 
gave the same result. 
38. Curved Surf aces are required for producing the coloured Ap- 
pearances at present under Consideration. 
It has already been seen, in the first part of this paper, that 
spherical curves give circular rings, and I have now shown 
that cylindrical forms produce streaks ; that a combination 
of spherical and cylindrical curvatures give elliptical rings, 
and that all sorts of variegated coloured phenomena are made 
visible by surfaces, which are irregularly and variously curved; 
these experiments prove in the fullest manner that the curva- 
ture of surfaces is the cause of the appearance, as well as of 
the shape of the coloured phenomena which are produced. For 
if we can invariably predict, from the nature of the curves we 
employ in an experiment, what will be the appearance and 
form of the colours that will be seen, it certainly must prove 
the efficacy of these curvatures in the production of such phe- 
nomena. This will receive additional confirmation in the fol- 
lowing article, which shows that 
