256 Dr. Herschf.l’s Experiments for Investigating 
bow to which it belongs, is proved by the characteristic 
colours of the streaks. In the blue bow-streaks, the blue 
colour is greatly predominant ; and in the streaks of the red 
bow, the red and green are most abundant, which could 
never happen if the reflective separation did not copiously 
furnish the blue, and the intromissive separation as copiously 
the red and green colours ; and what must put this dependance 
past all doubt, is the sudden change which may be made in 
the colour of the streaks ' for by the mere interposition of 
a screen, or by lifting up the prism towards the light, we 
may not only change one bow into the other, as has been 
proved in the 43d article, but when a plain surface is held 
under the base of the prism, in order to turn the bow into 
streaks, we may then change the colours of the streaks be- 
longing to one bow, into those which belong to the streaks 
of the other, with as much certainty as we can change one 
bow into the other. 
A beautiful experiment to prove this, is as follows. Let 
two equilateral prisms be tied together as in figure 8. Then 
standing at the distance of five or six feet from an open win- 
dow, with the prism held in the situation as represented, the 
sides b d, d c being covered with a pasteboard screen ef g, 
look into the side a b, straight forward to the window, and 
you will see beautiful blue bow-streaks. The rays which 
produce them enter through a c, are critically separated by 
reflection from the surface b c, and cause the blue bow, which 
by the plain surface b c, of the prism b d c, is converted into 
streaks that go to the eye at E. 
Without altering either the position of the eye, or of the 
prism, drop the screen efg, and the blue bow-streaks will 
