UPON THE DIFFERENTLY COLOURED RAYS OF LIGHT. S 



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 



