120 The Rev. Professor O'Brien on the Effect of 



perceived, but will affect the eye in some peculiar manner, 

 which may possibly give the light (supposing it to be a pure 

 colour of the spectrum) an appearance different from that of 

 any of the prismatic colours. 



Hence I think it is clear that the light represented by the 

 formula (1.) will appear to be a pure prismatic green, inasmuch 

 as the intensity goes through all its values 43,500000,000000 

 in one second, which number is considerably less than 

 458,000000,000000, being about the tenth part of it. 



34. If, instead of yellow and blue, we mix together a red 

 and violet, we shall find 



^^^^ = 134,500000,000000, 

 2 



which is between the 3rd and 4th part of 458,000000,000000. 



We also find 



^^-^ = 592,500000,000000, 

 2 



which is the number of undulations corresponding to the 



greenish-blue part of the spectrum. In this case the value of 



— - — being four times what it was in the former case, the 



compound colour will probably have a peculiar cast. 



35. It may be easily shown that if the two colours be mixed 

 in unequal intensities, similar consequences will follow, only 

 the tint of the compound light will be nearer to that of the 

 strongest component; but the consideration of this, and of 

 the mixture of more than two colours, we shall reserve for a 

 future communication. I shall only observe here, with refer- 

 ence to the experimental test of the truth of this explanation, 

 that the colours in certain parts of the purest spectrum are 

 probably mixed colours, and the above explanation applies 

 only to two perfectly pure and simple colours. 



On the Effect of Blue Glass upon the Prismatic Spectrum. 



36. When the prismatic spectrum is viewed through a plate 

 of common blue glass of proper thickness, most curious and 

 perplexing phaenomena present themselves, as is well known 

 (see the article " Light " in the Encyclopedia Metropolitana). 

 From these phaenomena it appears, that two decidedly differ- 

 ent colours (yellow and red) have almost the same degree of 

 refrangibility, and that two colpurs apparently identical (two 

 reds) have decidedly different degrees of refrangibility. We 

 shall now attempt to account for these facts. 



We have shown, in article 21, the connection between the 

 length of the wave and the colour, and it follows from what is 

 there proved (as we shall show more particularly hereafter) 



