C 259 3 
XVII. Continuation of Experiments for investigating the Cause of 
coloured concentric Rings , and other Appearances of a similar 
Nature. By William Herschel, LL. D. F. R. $. 
Read March 23, 1809. 
In the first part of this paper, I have pointed out a variety of 
methods that will give us coloured concentric rings between 
two glasses of a proper figure applied to each other, and it 
has been proved that only two surfaces, namely, those that 
are in contact with each other, are essential to their formation ; 
it will now be necessary to enlarge the field of prismatic phe- 
nomena, by showing that their appearance in the shape of 
rings has been owing to our having only used spherical curves 
to produce them. 
35. Cylindrical Curves produce Streaks. 
As soon as it occurred to me, that the cause of the figure 
of any certain prismatic appearance must be looked for in the 
nature of the curvature of one or both of the surfaces, that 
are essential to its production, I was prepared to expect that 
if a spherical curve, when applied to a plain surface of glass, 
produces coloured rings, a cylindrical one applied to the same 
would give coloured lines or streaks. To put this to the proof 
of an experiment, I ground one side of a plate of glass into a 
cylindrical curve, and after having given it a polish, I laid a slip 
of plain glass upon it, and soon perceived a beautiful set of 
