158 Oji the Nein} Analysis of Solar Light. 



produced; but merely tells us that the qualities of the colours 

 were unchanged. Sir John Herschel, as we have seen, made 

 the experiment with the same glass carefully, and describes 

 the results of it minutely ; and I have done the same ; and 

 both of us have found that the qualities of the colotirs voere 

 decidedly changed. But Mr. Air}' may perhaps have made 

 the experiment more skilfully than we did ; and he takes credit 

 for comparing the modified with the unmodified spectrum, and 

 with excluding all extraneous light, — precautions which others 

 have taken far more carefull}' and effectually than he did. With 

 all these precautions, however, he neglected the most important. 

 He received his s})ectrum upon a paper screc?i 1 and while he 

 was viewing it, his retina was influenced by all the various 

 colours which shone in his modified and unmodified spectrum. 

 In the experiments of Sir John Herschel, I believe, but cer- 

 tainly in my own, not a ray of light entered the room but what 

 passed through the narrow slit; the retina xvas our screen, 

 and the absorbing medium was held close to the eye and im- 

 mediately behind the prism, the only method of obtaining the 

 purest spectrum from a given prism. 1 have now before me 

 a modified and an unmodified spectrum. I can compare any 

 portion of the one with any portion of the other, or I can 

 examine it alone ; and so completely are the colours changed 

 with the combination of absorbents which I employ, that I 

 should regard the person as colour-blind who does not see the 

 change. In the striped spectrum which I have described in 

 my paper, the phaenomena are still more beautiful and in- 

 structive. 



But it is not merely by absorbent substances that the qua- 

 lities of the spectral colours may be changed. I have obtained, 

 during a series of unpublished experiments, similar effects from 

 the interference of pencils transmitted through perfectly colour- 

 less media, and prismatically analysed. 



What, then, is the result of all this discussion? Sir John 

 Herschel and I have minutely described certain experiments, 

 in which we have proved to our own satisfaction and that of 

 many others, that absorbing media change the colours of por- 

 tions of the spectrum. Mr. Air}-, by inferior methods as I 

 think, but by superior methods as he diinks, recollects having, 

 fourteen years ago, found that these colours were not changed ! 

 The Master of Trinity records the last of these results as un- 

 doubted scientific truth, and strives to transmit it to the latest 

 posterity. 



St. Leonard's College, St. Andrews, 

 February 5, 1847. 



