2 68 Dr. Herschel's Experiments for investigating 
different refrangibility. The angles at which the rays that 
constitute the blue bow are separated from the rest, may very 
properly be called critical , and the effect, which is the conse- 
quence of the oblique incidences that have been given, may 
with equal propriety be called a critical separation of the diffe- 
rently coloured rays of light. 
42. Account of a prismatic red Bozv. 
I must now introduce a prismatic appearance, which on ac- 
count of its similarity with the Newtonian blue bow, from 
which it only differs in colour, I have called, a prismatic red 
bow. It consists of red, orange, yellow, and some green rays ; 
and the red colour being upon the whole very predominant, 
it may not improperly be called a red bow. It is not produced 
by the Newtonian different reflexibility of the differently co- 
loured rays of light, but owes its origin to a modification which 
takes effect at the outside of the prism at very oblique angles 
of incidence, and may be called a different intromissibility ; 
but this, like the Newtonian different reflexibility, is only the 
consequence of the different refrangibility of light. 
To see the red bow, an observer should place himself in 
the open air, and standing with his back within a few feet of 
some wall or building, hold the side of an equilateral prism 
flat over his eyes, and look upwards to an altitude of about 
go 0 at the heavens ; he will then see a beautiful arch of a deep 
red colour, succeeded by a bright orange and yellow, with a 
considerable portion of green on the inside. The comparative 
darkness of the building behind will show the light in front 
to the best advantage. It is also to be observed, that all 
