2^4 Dr. Herschei/s Experiments jor investigating 
below the place where the bows can be seen, which would not 
have happened had a plain glass been used instead of the 
prism ; for with such an arrangement, coloured rings may 
be seen at the most oblique as well as perpendicular stations 
of the eye. — As soon as the blue bow is perceived, the rings 
begin to be formed, first partly, then half, and lastly, we see 
them completed; and what is remarkable, these coloured rings 
are of such a magnitude and brightness, that they cannot be 
a moment mistaken for those we see when a plain glass is 
laid upon the same spherical curve. — The eye being then gra- 
dually elevated above the range in which the bows may be 
seen, these rings will pretty suddenly shrink in their dimen- 
sions, and lose much of their brilliancy ; till at last, when the 
eye comes to a perpendicular situation, we find them dwindled 
away to the size and appearance of such as. may be seen when 
a plain glass is substituted for the prism. 
Irregular surfaces are no less decisive in the phenomena they 
exhibit ; for when an equilateral prism is laid upon red mica 
in a strong illumination of scattered light, we may see a most 
admirable variety of very minute coloured appearances, when- 
ever the eye is brought to the blue bow place ; but as soon 
as it is in the least elevated above, or depressed below that 
situation, these fantastical figures are sure to vanish. 
51. A Lens may be looked upon as a Prism bent round in a 
circular Form. 
Those who have followed me in the analysis of the blue 
and red bows, will readily enter into the application I shall 
make of this theory to the generation of coloured rings by 
lenses. 
