New Analysis of Solar Light. 403 



the view of Brewster, The first dwelt particularly upou the fact 

 that Brewster, in the method which he applied, had not the 

 colours changed by absorption simultaneously with the unchanged 

 colours before him, and could therefore readily make a mistake 

 in the comparison. To this Brewster replied, and I can corro- 

 borate his statement by my own exjierience, that in his observa- 

 tions the changes of colour were for the most part sufficiently 

 striking to be observed without difficulty. Draper and Melloni 

 expressed their doubts regarding the purity of the spectrum used 

 by Brewster, and thought that the single colours might overlap 

 each other considerably. From Brewster's statements in reply 

 to these attacks, it is e\ident that no such overlapping of the 

 colours took place; his later experiments on the number of 

 Fraunhofer's lines in the solar spectrum, show further that he 

 possessed far more complete apparatus for the separation of the 

 rays of different refrangibility than even Fraunhofer himself, or 

 perhaps any other physicist. A careful repetition of at least the 

 most important of his experiments, carried out in exact accord- 

 ance with his method, and with every precaution hitherto deemed 

 necessary, has indeed taught me that the facts which he affirms 

 to have observed are described with perfect accuracy; indeed 

 nothing else could be expected from so skilful an observer; I 

 trast, however, to be able to show that his explanation of these 

 experiments is untenable, and thus to remove the apparent con- 

 tradiction of the views of Newton. 



It is much to be regretted that Brewster nowhere gives a de- 

 tailed description of his method of observation. Hence it is that 

 Draper and Melloni might do him injusticeby their suppositions, 

 and hence also, that at the outset I must ask his indulgence in 

 case I also should allude to possible sources of error which he 

 has taken pains to avoid. Partly from his replies to his anta- 

 gonists, and partly from the description in his Treatise on Optics, 

 London, 1831, I gleaned the following regarding his mode of 

 observation. In the shutter of a dark room he made a narrow 

 aj)erture, and looked at this \\ith the naked eye through a strongly 

 refracting prism ; in the s])cctrum thus formed he was able to 

 detect the stronger lines of Fraunhofer. He then introduced 

 the absorbing coloured medium between the eye and the prism, 

 and observed tlie altered spectrum. Besides this, he repeated 

 the experiment with spectra in which a number of dark bands 

 were formed by interference, and the different colours separated 

 in a still more evident manner. Brewster does not state whether 

 it was direct sunlight, or merely the reflected light of the sky 

 which he permitted to enter through the aperture and fall upon 

 the prism. We must, however, assume that in most cases he 

 made use of the former ; for when the slit is sufficiently narrow 

 2D2 



