170 Dr. Herschel’s Experiments for Investigating 
of a surface upon which they fall after their separation, i need 
but refer to the experiment which has been mentioned in the 
5th paragraph of the ^2d article, where the rays of the bows, 
which can only be seen in one particular situation, when they 
come directly to the eye, are effectually rendered visible in 
every direction, by the scattering power of the surface on 
which they are thrown. 
I proceed now, by different decisive experiments to prove, 
that the objection in neither of the senses it may be taken, 
can affect the validity of the theory I have explained. 
First set of experiments. Having ground and polished a 
metalline mirror, to the convexity of a sphere of 40 feet dia- 
meter, I laid upon it a right angled prism, and when they 
were properly exposed to the light, I lifted the eye gradually 
up to the blue bow place, and saw the rings that were formed 
of the colours critically separated by the base of the prism. 
That these rings owe their formation to the joint effect of the 
critical separation, and modifying power of the spherical metal- 
line reflecting surface, cannot, after what has been proved in 
the 56'th article, admit of a doubt. I then lifted the eye very 
slowly higher and higher, till it was brought to the vertex of 
the prism, and attending minutely to the rings all this time, I 
could no where perceive the least interruption in their uni- 
form visibility. I do not take notice of the gradual changes 
in the colour and size of the rings, because such gradual 
changes equally happen to those that are seen between object 
glasses. When the eye is over the vertex, the prism being 
equiangular at the base, we see in the opposite side an equal 
set of rings. We may then advance the eye still farther, 
and keep the first set in view, or, what will be more convenient, 
