154> Sir D. Brewster's Iteply to the Astronomer Royal 



has invented to designate the unfortunate members of the 

 colour-blind community. 



But before I proceed to give a notice of the analysis of the 

 spectrum by absorption, I must refer to the mode of making 

 the experiments. Mr. Airy says that ^Hhe eye has no memory 

 for colours" and that it is therefore necessary to compare im- 

 mediately the modijied with the iinmodificd spectrum. It may 

 be true that the eye has no memory of any kind, and there- 

 fore not for colours ; but I know that I have a memory for 

 colours, and that the colours are so much committed to my 

 memory by fifty years' schooling, that without any immediate 

 comparison I can tell whether or not the green space has 

 become yelloiser, and whether or not, by certain combinations 

 of absorbing media, I see before me a portion of 'iSihite light 

 standing close to a portion oi' red light in the spectrum, like 

 an almond and a cherry stuck together. 



But, apart from this consideration, I beg to remind Mr. 

 Airy that I was the first person to use the methods which he 

 considers his of placing the modijied beside the unmodijied spec- 

 trum, by employing the same slit for both; and that I showed 

 it to \\\mjifteen years ago, when he did me the honour of paying 

 me a visit in Roxburghshire. In my paper On the Lines of the 

 Spectrum, read to the Royal Society in 18.33*, I write thus, 

 when speaking of the coincidence of the Nitrous gas lines with 

 those of Fraunhofer : — 



" In order to afford ocular demonstration of this fact, I formed the solar 



and the gaseous spectrum luith light passing throttgh the same aperture, so 

 that the lines in the one stood opposite those on the other, like the divi- 

 sions on the vernier and the limb of a circle, and their coincidence or non- 

 coincidence became a matter of simple observation. I then superimposed 

 the two spectra when they were both formed by solar light, and thus ex- 

 hibited at once the tvvo series oflines, with all their coincidences and all 

 their apparent deviations from it. Professor Airy, to whom I showed tiiis 

 experiment, remarked that he saw the one set of lines through the other, 

 which is an accurate description of a phicnonienoM perhaps one of the 

 most splendid in physical optics, whether we consider it as appealing to the 

 eye or to the judgement." 



On the 15th of April 1822, I laid before the Royal Society 

 of Edinbm'gh a paper, in which I describe specific experi- 

 ments, proving that the colour of parts of the spectrum is 

 changed by absorbing media, and that a distinct yellow band 

 can be insulated on the most refrangible side of the line Df. 

 About the same time Sir John Herschel addressed a letter to 

 me On the Absorption of Light by Coloured Media, &c.,:{: 



• Edinburgh Transactions, vol. xii. p. 525. 

 ■I- Ibid. vol. ix. p. 433-444. 

 X Ibid. pp. 445— 460. 



