200 Dr. HerscheFs Experiments for investigating 
coloured streaks. The broadest of them was at the line of 
contact, and on each side they were gradually narrower and 
less bright. The colours in the streaks were similar to those 
in the rings, and they were in the same manner changeable 
by pressure as in them. Their order was likewise the same, 
if we reckon from the line of contact, as with rings we do 
from the center ; so that these streaks differed in no respect 
from rings, except in their linear instead of circular arrange- 
ment. 
When the cylindrical surface was laid upon a plain slip of 
glass, the same streaks were seen as in the former experi- 
ment. They were of a lively red and green colour, and I saw 
at least ten, eleven, or twelve on each side of the line of 
contact. 
Metalline surfaces had the same effect, for when the cylin- 
drical surface of glass was laid on a plain metalline mirror, I 
had red, orange, yellow, green, and blue streaks. In the 
same manner a plain slip of glass placed upon a polished 
part of a brass cylinder of 3^-inch in diameter, produced also 
coloured streaks. 
The combination of two cylindrical surfaces has an effect 
on the streaks, which is similar to that which the contact of 
two spherical ones has on the rings ; for when I placed the 
cylindrical surface of glass longitudinally upon the polished 
part of the brass cylinder, the streaks were contracted as rings 
would have been by the application of two spherical curves to 
each other. 
