420 Professor Forbes on the Colours of the Atviosphere. 



mical collections, and periodical works, respecting it. The 

 most copious I'eferences I have found amongst German 

 authors, but these 1 have, in almost every case, been able 

 to verify by a reference to the original authorities. The re- 

 sult has been a reduction to a few of those authors who have 

 added anything of consequence to a subject which has rather 

 been one of opinion than of science, until lately ; and to still 

 fewer of those who, by any one original observation or expe- 

 riment, have added a single mite to the data for reasoning. 

 The mass of copyists I may pass over in silence, or with little 

 notice, and thus I hope to be able to reduce into moderate 

 compass the results of a considerably tedious investigation. 



It is impossible to advance any consistent theory about the 

 colours of dawn, sunset, and clouds generally, without inclu- 

 ding 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 themixtureof the white solar 

 light reflected from the matter of the atmosphere, with the in- 

 tense darkness of the celestial spaces beyond. This doctrine 

 was also maintained by Fromond, and later by De la Hire, 

 Funk, Wolff, and Musschenbroek, after the Newtonian theory 

 of colours should have banished such reasonings from science. 

 It was still later i-evived, to the disgrace of modern physics, 

 amongst the chromatic fancies of Gcithef. Otto Guericke 

 liad nearly similar views. 



The first trace of a more reasonable docti'ine 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 observations §. 

 In opposition to the doctrines of Fromond, Fabri attributes 

 the colour of the sky to the reflection of hght, by corpuscular 

 particles floating in the atmosphere ; 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 cus- 

 tomary modesty, rather in the form of suggestions than asser- 



* Traite de la Peintiire, quoted in Gehler's Worterbuch, art. Atmosphare, 



■\ Farbenlehre, i. 59, quoted by Humboldt. 



t Eberhard in Rozier, i. 620. 



§ Fabri's Dialogues (1669), of which I have found a copy in the Advo- 

 cates' 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. 



II " On peut croire qu'ii y a des couleurs primitives dans qnelques corps, 



comme du bleu dans I'air II semble qu'il y ait du yard dans 



I'eau." — Mariotte, CEicvres, i. 399. Leide 1717. 



