2C)8 Dr. Herschel’s Experiments for investigating 
surfaces, it may be remembered that when in the last para- 
graph of the- 39th article I said that streaks could not be seen 
by laying another plain glass under it, I intimated at the same 
time the formation of colours ; this will now admit of a satis- 
factory explanation. Scattered rays will enter into a parallel 
piece of glass, and by reflection the critical separation of 
colours will take place on its interior surface, so that if this 
effect could be seen, a blue bow would appear ; and in the 
same manner a red bow might be seen by rays intromitted 
through the lowest surface. In consequence of the course of 
these rays, streaks would also appear from each of the bows 
when another plain glass is laid under the parallel piece ; but 
from a calculation made according to the principles that have 
been established in the preceding part of this paper, the reflection 
of a mean ray of the blue bow from the interior surface being 
at the angle 49 0 57' 3'' ,3 ; and this being also the oblique in- 
cidence on the upper surface, a ray which comes in that direc- 
tion with the mean refrangibility of the rays of the blue bow 
cannot come out of glass. The angle of obliquity of the mean 
intromitted ray for the red bow is 49°38' 19", 5, and on com- 
puting its direction by the mean refrangibility of the red bow, 
it will also be found that it cannot clear the glass. I have seen 
the bows and their streaks when the upper surface of the 
glass v/ as inclined only nine degrees to the lower one ; and 
possibly a much smaller angle would have been sufficient to 
permit the emergence of the coloured rays. The strong re- 
flection from the outside of the glass, and the contraction of 
the dimensions of the bows are however much against per- 
ceiving them at a great obliquity. 
