63S ACCOUNT OF SOME CASES OF THE PRODUCTIOy OF COLOURS. 



green, and violet, for red, yellow, and, blue, 

 aud the numbers 7, G, and 5, for 8, 7, and 6. 

 The same prismatic analysis of the colours 

 of thin plates appears to furnish a satisfac- 

 tory explanation of the subdivision of the 

 light of the lower part of a candle : for, in 

 fact, the light, transmitted tlirough every part 

 of a thin plates is divided in a similar man- 

 ner into distinct portions, increasing in num- 

 ber with the thickness of the plate, until they 

 become too minute to be visible. At the 

 thickness corresponding to the ninth or tenth 

 portion of red hght, the number of portions 

 of different colours is five ; and their pro- 

 portions, as exhibited by refraction, are 

 nearly the same as in the light of a candle, 

 the violet being the broadest. We have only 

 to suppose each particle of tallow to be, at 

 its first evaporation, of such dimensions iis to 

 produce the same effect as the thin plate of 

 air at this point, where it is about —o Jts^ of 

 an inch in thickness, and to reflect, or per- 

 haps rather to transmit, the mixed light pro- 

 duced by the incipient combustion around it, 

 and we shall have a light completely resem- 

 blinsthat which Dr. VVollaston has observed. 

 There appears to be also a fine line of strong 

 yellow light, separate from the general spec- 

 trum, prin(;ipal!y derived from the most su- 

 perficial combustion at the margin of the 

 flame, iind increasing in quantity as the 

 flame ascends. This yellow light is rendered 

 much more conspicuous by putting a few 

 grains of salt ou the wick of the candle, and 

 it is, not improbably, always derived from 

 some salt contained in the tallow. Similar 

 circumstances might undoubtedly be found 

 ju other cases of the production or modifi- 



caiion of light ; and experiments upon this 

 subject might te«d greatly to establish the 

 Newtonian opinion, that the colours of all 

 natural bodies are similar in their origin to 

 those of thin plates ; an opinion which ap- 

 pears to do the highest honour to the saga- 

 city of its author, and indeed to form a very 

 considerable step in our advances towards an 

 acquaintance with the intimate constitution 

 and arrangement of material substances. 



I have lately had an opportunity of con- 

 firming my former observations on the dis- 

 persive powers of the eye. 1 find that, at the 

 respective distances of 10 and 15 inches, the 

 extreme red and extreme violet rays are si- 

 milarly refracted, ■ the difference being ex- 

 pressed by a focal length of 30 laches. Now 

 the interval between red and yellow;!* about 

 one fourth of the whoJe spectrum; conse- 

 quently, a focal length of 120 inches ex- 

 presses a power equivalent to the dispersion 

 of the red and yellow, and this differs but 

 little from 132, which was the result of the 

 observation already described. • I do not 

 know that the^e experiments are more accu- 

 rate than the former one ; but I have repeated 

 them several times under different circum- 

 stances, and I have no doubt that the dis- 

 persion of coloured light in the human eye is 

 nearly such as I have stated it. It may also 

 be ascertained very accurately, by looking, 

 through an aperture of known dimensions, at 

 the image of a point dilated by a prism into 

 a spectrum, and measuring the angle formed 

 by its sides on account of the difference of 

 refrangibility of the rays ; and this method 

 seems to indicate a greater dispersive power 

 than the former. 



