THE OPTICAL PHENOMENA OP THE ATMOSPHERE, 
217 
writers relative to the cause of the Rainbow ; many of them 
seem to have had an idea, that the refraction of the Slabs rays 
had something to do with it, but in what manner they do 
not appear to have known. 
There are frequently two bows seen — a primary and a 
secondary one : the former is by far the brightest, being 
formed by the rays of Light falling on the upper part of 
the drops of rain ; for a ray of Light entering the upper 
part of a drop of rain will, by refraction, be thrown upon 
the inner part of the spherical surface of that drop, where, 
undergoing a second refraction, it will be sent towards the eye 
of the spectator ; since the rays which fall upon the primary 
bow come to the eye after two refractions and one reflection, 
and the colours of this bow, reckoning’ outwards, are violet, 
indigo, blue, green, yellow, orange, and red. The secondary 
bow is formed by the rays of Light falling on the lower parts 
of the drops of rain : these rays, like the former, undergo two 
refractions ; viz., when entering the drops of rain, and when 
emerging from them in passing to the eye ; but they suffer two 
or more reflections in the interior surface of the drops : hence 
the colours of these rays are not so strong and well-defined 
as those in the primary bow, and appear in an inverted order : 
red first, then orange, &c. This may be experimentally shown 
in the following manner : — Take a glass globe, filled with water, 
place it in the sunshine, turn your back to the Sun and view 
the globe, at such a distance that the part of it farthest from 
the Sun may appear of a full red colour ; then the rays which 
come from the globe will be found to make an angle of 42°, 
with the direct rays of the Sun : retain the eye in this position, 
and let another person gradually lower the globe ; then the red, 
orange, yellow, &c. colours will appear in succession, as in the 
primary bow. Again, if the glass globe be elevated, so that the 
side nearest the Hun may appear red, then the rays which come 
from the globe will be found to make an angle of about 50° 
with the direct rays of the Sun : retain the eye in this position, 
and let another person gradually raise the globe ; then the rays 
mil successively change from red to orange, green, &c., as in 
the secondary bow.* 
The breadth of a primary bow is about 1° 45', and of a 
secondary one, 3° 10'. From theory we may be led to infer the 
existence of tertiary and quaternary bows, but owing to their 
position and faintness, they are usually invisible. In addition to 
the regular bows, supernumerary ones are occasionally seen, 
which depend for their development on certain peculiar con- 
ditions of the atmosphere. Lunar Rainbows may also be 
* This illustration is, we believe, due to Keith. 
Q 2 
