UPON: THE DIFFERENTLY COLOURED RAYS OF LIGHT. 3 
. In order to observe the secondary spectrum with distinctness, 
a prism of sulphuric or phosphoric acid should be made to act 
in opposition to a prism of flint-glass, or what is still better, 
to a prism of oil of cassia; the uncorrected fringes will in this 
case be remarkably broad and distinct, and I have seen them, 
when a prism of flint-glass acted in opposition to a prism of 
phosphoric acid, with a refracting angle of only 11°. When 
we look at the bars of a window through a prism of phospho- 
ric acid, they are fringed with the prismatic colours, and so dif- 
ferent are these colours, in their general appearance, from the 
colours formed by a flint-glass prism, that any person unac- 
quainted with the subject, would immediately perceive that 
there was an excess in the space occupied by the orange-co- 
loured light in the spectrum formed by the phosphoric acid. 
This simple experiment, may be considered as affording ocular 
evidence of the irrationality of the coloured spaces. 
In my Treatise on New Philosophical Instruments, I have 
already published the first experiments which I made upon 
this subject, and I have there pointed out a method of ob- 
taining a numerical value of the magnitude of the second- 
ary spectrum*. Since these experiments were published, I 
have pursued the subject to a much greater length, and have 
examined almost every transparent body of importance. The 
results of both these sets of experiments are contained in the 
following pages, and are arranged in an alphabetical order, to 
facilitate the reference from the General Table given at the end 
of the Paper. 
* The reader is referred to this work, p.353—401. for farther details illus: 
trative of this subject. 
A@2 1. Acetate 
