270 Dr. Herschel’s Experiments for investigating 
whole angle of the red bow is 15' 4b", 1, and the mean 
obliquity of the eye at H is 49° 38' i9 // ,5. 
In the calculation of both the bows, the situation of the eye 
at H has been determined, as it would be, were the rays to 
remain in glass ; but as they will be refracted by the side of 
a prism, when they come out of it, proper computations must 
be made not only of the place of the eye in air, but also 
of the angle which the bow will subtend ; for this will be 
found to be considerably different in different prisms ; those 
that have large refracting angles will magnify the bows more, 
and require the eye to be nearer than others that have smaller 
angles. 
These bows may be examined at leisure, by projecting them 
upon a white ground in the following manner : 
In a dark room, by a reflecting apparatus, I admitted a 
horizontal beam of the solar light through an opening of 
about an inch and a half in diameter. The formation of the 
bows requiring scattered light, * I covered the opening with a 
piece of glass evenly roughned on both sides. Then, with an 
intention to obtain a projection of the blue bow, I placed a 
prism having one angle of 91 0 and the other two nearly equal, 
close to the emeried surface, and turned it upon its axis till 
the angle of obliquity of the scattered rays, that fell on one 
side of the prism, was proper for the required critical separa- 
tion of the coloured rays. The obliquity of the middle ray 
with the base, for this purpose, it has been shown, must be 
49 0 57' 3", 3. In this position the interior critical separation of 
the prismatic colours taking place, the blue part, namely the 
violet, indigo, blue, and about one half of the green rays were 
* See the first paragraph of the 46th article of this paper. 
