NEWTONIAN PRISM BUT THREE ORIGINAL RAYS. 13 



vanity; a lesson so powerful on the subject of the insufficiency 

 of human understanding, that it should teach us humility. On 

 this point it is philosophy to be silent, and to bow with sub- 

 mission to that Omnipotent Power by whose WILL light was 

 formed a*nd we are in being; but we may fairly investigate 

 the effects of light, and judge of its nature as a body, by 

 direct evidence, and by. analogy, with the hope to elicit some 

 benefit. 



The triangular prism of Newton displays in its usual appli- 

 cation, so universally known, that beautiful spectrum, which 

 was divided by that eminent philosopher into what are gene- 

 rally termed the seven prismatic colored rays, (See Plate I, 

 No. 1 3 ) and which were for a long time supposed to be distinct 

 and homogeneous portions of refracted light : these were after- 

 wards reduced to four by Dr. Wollaston. 



It has been, however, fully demonstrated that there are but 

 THREE original homogeneous colored rays, red, yellow, and 

 Hue, by Sir David Brewster, whose investigations on the sub- 

 ject of light and its properties, unceasing and indefatigable for 

 years, have been rewarded by his discovery of many most valu- 

 able and interesting phenomena, recorded in the works, of that 

 eminent philosopher, as well as in the various Transactions of 

 the learned and scientific Societies. 



It must be evident to an observer viewing the Newtonian 

 spectrum, that except the red, yellow, and blue, all the other 

 rays are not homogeneous ; that when the yellow ray overlaps 

 the red ray, it is gradually acquiring an intensity of tints of 

 light-orange, dark-orange, and scarlet, till it terminates by an 

 exposure of the red ray distinct ; and that where the blue 

 overlaps the yellow, it is deep-green, light-green, and very 

 pale-green terminating in yellow ; on the other side, the blue is 

 first converted into deep or intense purple, and gradually 

 softens down into violet, proving the want of distinct homo- 

 geneous intensity in all the rays except the primary red, 

 yellow, and blue. 



