492 Sir David Brewster, in Reply to M. Melloni, 



yellow and green rays whicli compose it must liave the same 

 refrangibility as I maintain tliey have. W\t\s yellow or green^ 

 why not say so? This deep indistinct colour is, doubtless, 

 neiihev yellow nor green. We believe, or rather we conjecture 

 (for we cannot speak with certainty as we do not know pre- 

 cisely what glass he used), tlrat it is a gray, that is, a dirty 

 white, consisting of red, blue and yelloxv light, not in such 

 proportions as to make w/iite light. 



We shall admit, however, for the sake of argument, that 

 our author's experiments are perfectly correct, and that there 

 has been an overlapping of rays in the spectrum which I em- 

 ployed. The admission would not in the slightest degree 

 affect my analysis of the spectrum. M. Melloni and Dr. 

 Draper cannot surely have read my original memoir in the 

 Edinburgh Transactions. Do they know that I have insulated 

 a wide iieam of white light in the most luminous part of the 

 spectrum ? Do they know that I have done this in spectra 

 divided by interference into dark and luminous portions where 

 there are no lateral rays to invade the portion subjected to 

 absorption ? Do they know that I have found green light near 

 the line C of Fraunhofer, and considerably within the red 

 space? Have they repeated these experiments, or do they 

 possess an apparatus fitted for their repetition? I believe they 

 have not; and I am persuaded that the experiments described 

 in the memoir referred to have not been repeated by any living 

 philosopher. 



'Although I feel no disposition to adduce any new argu- 

 ments in support of opinions which I consider as beyond the 

 reach of challenge, I am, nevertheless, desirous of stating some 

 facts, as observed by others, which may influence the views 

 of those who are not qualified, or who may not have the desire, 

 to investigate the subject experimentally. 



Dr. Wollaston, in his elegant examination of the spectrum 

 produced from the light of the sky with an aperture of one- 

 twentieth of an inch, found only four colours, red, yellowish- 

 green, blue and violet. He saw no yellow. Dr. Thomas 

 Young informs us that " he repeated Dr. WoUaston's very 

 interesting experiments with perfect success." He calls Dr. 

 WoUaston's description of the spectrum the " correction of 

 the description of the spectrum ;" and he modifies his own 

 theoretical views by substituting red and green in place of 

 red, orange and yellow, when treating of the colours which 

 compose the less refrangible portion of the spectrum*. In 



* Philosophical Transactions, 1802; or Lect. on Nat. Phil., vol. ii. 

 pp. 637, 638. 



