158 Dr. Herschel’s Experiments for Investigating 
comes to the altitude of the bow, a set of rings will be seen, 
whose colours, when the bow goes across their center, will be 
red and green. Some motion of the eye to bring the bow a 
very little above or below the center, will show the colours 
to advantage, and in this position of the eye, we are sure to 
see the rings precisely in the range of the bow which is 
turned into rings, but remains visible at both sides, where the 
critical separation is known to take place. When they have 
been sufficiently viewed, let the screen be removed, that the 
brighter light of the heavens from above, may transform the 
red bow into a blue one, which will at the same time instantly 
change the colours of the rings from red and green to blue. 
If it should now be alledged, that streaks or rings may still 
be independant of critical separations, notwithstanding their 
taking the colours of the bows, because they must necessarily 
appear blue, red, or green, when they are seen in rays of 
these colours, we may answer this objection by proving ex- 
perimentally, that any adventitious colours that may occa- 
sionally mix with streaks or rings, can only tinge them in 
the places where they pass to the eye in the same direction, 
but can themselves not produce either blue, red, or green 
streaks or rings. Place yourself before a window, and hold- 
ing in your hand a right angled prism, with a plain slip of 
glass under the base, look in at one side, and turn the prism 
upon its axis, till you see the horizontal bars of the window 
tinged with blue above, and red below ; bring the red bow- 
streaks upon one of these bars, and lowering the streaks 
gradually, you will find that the colours of the bar, merely 
affect only those parts of the streaks over which they pass, 
but do not cause any additional streaks of their own colour. 
