292 Dr. Herschel’s Experiments for investigating 
is merely the consequence of the position of the eye, and the 
modifying power of the surface through which it sees them* 
For a proof of this, it would be sufficient to refer to the prin- 
ciples of the formation of the bows, from which it must be 
evident that the critical separation of the rays will be exerted 
in every direction, and that the extent of the bows we see 
would consequently be parallel to the sides and base of the 
prism, if the eye could receive the rays which form them, 
every where in the same angle from a line drawn parallel to 
that side of the prism through which they pass. An experi- 
mental confirmation of this we have by laying down a prism, 
and keeping either of the bows in view while we gradually 
draw the eye away; it will then be seen that the curvature, 
which the bows had assumed, will continually be diminished, 
and nearly vanish at a very moderate distance. 
50. The Colours of the Bow-streaks owe their Production to the 
Principle of the critical Separation of the different Parts of the 
prismatic Spectrum. 
That streaks will be produced when a plain glass is laid 
under the side of a prism which forms either of the coloured 
bows, has already been sufficiently shown ; but that these 
streaks, as well as the rest of the phenomena which have been 
mentioned in the 44th article, are exclusively to be deduced 
from the same principle by which the bows have been ex- 
plained will require some proof. With regard to streaks, the 
following experiment, I believe, will remove every doubt upon 
the subject. 
Let a plain glass be laid under the base of a right angled 
prism ; then, if the eye at first be placed very low, no streaks 
