1887.] Captain W. de W. Ahney on Sunlight Colours. 61 



WEEKLY EVENING MEETING, 



Friday, February 25, 1887. 



William Huggins, Esq. D.C.L. LL.D. F.R.S. Manager and Vice- 

 President, in the Cliair. 



Captain W. de W. Abney, R.E. F.R.S. 3LB.L 



Sunlight Colours. 



Sunlight is so intimately woven np with our physical enjoyment of 

 life that it is perhaps not the most uninteresting subject that can be 

 chosen for what is — perhaps somewhat pedantically — termed a Friday 

 evening " discourse." Now, no discourse ought to be possible without 

 a text on which to hang one's words, and I think I found a suitable 

 one when walking with an artist friend from South Kensington 

 Museum the other day. The sun appeared like a red disk through 

 one of those fogs which the east wind had brought, and I hajtpened 

 to point it out to him. He looked, and said, " Why is it that the sun 

 appears so red ? " Being near the^ railway station, whither he was 

 bound, I had no time to enter into the subject, but said if he would 

 come to the Royal Institution this evening I would endeavour to 

 explain the matter. I am going to redeem that promise, and to 

 devote at all events a portion of the time allotted to me in answering 

 the question why the sun appears red in a fog. I must first of all 

 appeal to what every one w^ho frequents this theatre is so accustomed 

 to, viz. the spectrum ; I am going not to put it in the large and 

 splendid stripe of the most gorgeous colours before you with which 

 you are so well acquainted, but my spectrum will take a more modest 

 form of 23urer colours some twelve inches in length. 



I w^ould ask you to notice which colour is most luminous. I 

 think that no one will dispute that in the yellow we have the most 

 intense luminosity, and that it fades gradually in the red on the one 

 side and in the violet on the other. This then may be called a quali- 

 tative estimate of relative brightnesses ; but I wish now to introduce 

 to you what was novel last year, a quantitative method of measuring 

 the brightness of any part. 



Before doing this I must show you the diagram of the apparatus 

 which I shall employ in some of my experiments. 



R R are rays (Fig. 1) coming from the arc light, or, if we were 

 using sunlight, from a heliostat, and a solar image is formed by a lens, 

 Li,on the slit Sj of the collimator c. The parallel rays proluced by 

 the lens l^ are partially refracted and partially reflected. The former 

 pass through the prisms PiPo, and are focused to form a spectrum by 



