goo Dr. Herschel’s Experiments for investigating 
of the rays of light. Now, as this must be admitted; it will 
certainly not be philosophical to look for a different cause of 
the same or similar effects, when convex glasses, which have 
all the required prismatic properties are used to produce them.* 
To show the great similarity, or rather the identity of these 
effects, let us examine them in different points of view, and 
since the variety of the configurations is no longer an object 
that wants explaining, I shall only take the most simple case 
of each, namely, the coloured rings, that are produced when 
a piano*- convex lens is laid with its convex side upon a plain 
reflecting surface ; and the coloured streaks which are pro- 
duced when the base of a right angled prism is in the same 
manner placed upon such a surface. 
The form of rings arises from the spherical figure of the 
lens.-f- 
The right-lined appearance of the streaks is owing to the straight 
fgure of the plain surface of the prism f 
The colour of the rings may suddenly be changed. § 
The colour of the blue bow-streak may as instantly be converted 
into those of the red bow . || 
The cause of the sudden change of the rings has been shown 
to be that the sets of one colour are seen by reflection, and 
those of other by transmission.^ 
* Ey this it will be understood that if any case should occur, in which the critical 
separation cannot account for the observed phenomena, we are then authorised to look 
out for some other cause to explain them. 
f See the first paragraph of this paper. 
J See the 49th article. 
^ See the 15th article of the first part of this paper. 
H See the 43d article. 
f See the 8th article of the first part of this paper. 
