376 PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 



sunset, and clouds generally, without including the fact of the blue colour of the 

 sky. The first notice I find quoted on the subject by way of explanation, is 

 Leonardo da Vinci's,* who attributed it to the mixture of the white solar light re- 

 flected from the matter of the atmosphere, with the intense darkness of the celes- 

 tial spaces beyond. This doctrine was also maintained by Fromond, and later by 

 De la Hire, Funk, Wolff, and Musschenbroek, after the Newtonian theory of co- 

 lours should have banished such reasonings from science. It was still later re- 

 vived, to the disgrace of modern physics, amongst the chromatic fancies of 

 Gothe.f Otto Guericke had nearly similar views. 



The first trace of a more reasonable doctrine I find quoted from the writings 

 of HoNORATUs Fabri,:]: probably from his Optical Essays, published at Lyons in 

 1667, and which must, therefore, have been independent of Newton's observa- 

 tions. § In opposition to the doctrines of Froisiond, Fabri attributes the colour of 

 the sky to the reflection of light, by corpuscular particles floating in the atmo- 

 sphere ; and Mariotte, about the same time, seems boldly to have maintained that 

 the colour of air is blue. || 



Newton's thoughts on this subject are given, with his customary modesty, 

 rather in the form of suggestions than assertions ; and as many writers of the last 

 century have only reproduced his ideas with slight alterations, it is important to 

 observe his own exact statement of them. Newton's opinion respecting the co- 

 lours of natm-al bodies, whatever judgment we may form as to its universal ap- 

 plication, was singularly ingenious, and well worked out. He had discovered, in 

 the course of his memorable investigation on the colours of thin plates, that every 

 transparent body begins to reflect colours at a certain thickness ; that these vary 

 according to definite laws, as the thickness diminishes, passing through an im- 

 mense variety of compound tints, until at length it becomes so thin (as in the case 

 of the soap-bubble) as to be incapable of reflecting any colom* at aU : the last colour 

 it reflects being orange, yeUowish- white, and flnally blue, before they vanish ; 

 these are called colours of the first order. Now, on this subject, Newton says, 

 " The blue of the first order, though very faint and little, may possibly be the co- 

 lour of some substances ; and particularly the azure colour of the sky seems to be 

 of this order. For aU vapours, when they begin to condense and coagulate into 

 small parcels, become first of that bigness whereby such an azure must be re- 

 flected before they can constitute clouds of other colours. And so this being the 



* Traite de la Peinture, quoted in Gehler's Worterbuch, art. Atmosphdre. 



f Farbenlehre, i. 59, quoted by Humboldt. | Eberhard in Rozier, i. 620. 



§ Fabri's Dialogues (1669), of which I have found a copy in the Advocates' Library, contain 

 many allusions to the imperfect transparency of the air, and the foreign particles mixed with it ; but 

 I do not find his theory of the blue colour clearly stated. 



(1 " On peut croire qu'il y a des couleurs primitives dans quelques corps, comme du bleu dans I'air. 

 . .... 11 semble qu'il y ait du verd dans I'eau." — Mariotte, CEuvres, i. 299. Leide 17J7. 



