On the DecprnposilJon vf Light, 919 



To carry thi^ into execiuioa, I placed be^'orc the image • f 

 the spectrum, received on the cloth, a screen, bv which one 

 portion was concealed, and another lel't open. This screen, 

 however, was peribrdled bv a small hole, throngh which 

 passed a rav ot" coloured Ight belonging t) that part of the 

 epectrum which was concealed. Lasily, this little coloured 

 ray was received on a mttallic speculum placed between the 

 screen and the cloth, and incliiied so as to throw it on a 

 given point of that part of the spectruni whch ani;.'cd freely 

 on the cloth, 'i'hus the coio^.r ari^i.ng from the mixture of 

 two rays of light was ob'^erved. 



I varied the trial ol tliis apparatus on different points, the 

 corresponding tints ot which were diametrically opposite oa 

 tlie dial of colours. In several instances I did not obtain a 

 white free from all tint of colour, because a certain propor- 

 tion in the quantity as well as quality of the elements is ne- 

 cessary; but having carried the little image almost to the 

 limit of the green and blue, it gave a decided and bright 

 white when thrown on the extrcinity of the red. 



This very remarkable fact adds fresh support to my pro- 

 position respecting the compound slate of the colour in cer- 

 tain parts of the spectrum, simplitied to the utmost. Por, if 

 the combination of the three colours I have nientiuned be 

 necessary to produce white, as every thing tends to persuade 

 tis,we must admit the existence of violet in the greenish blue 

 with which the experiment was made. 



N. B. It may be necessary to add, in explanation of fig. 2. 

 Plate IV., that the colours of the three circles arc distin- 

 guished by dotted lines; the red by round and long points 

 alternately ; the green by one long point and two round ; 

 the violet by ont long and three round. 



XXXI. The 



