Professor Forbes on the Colours of the Atmosphere. 421 



lions; 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 colours of natural bodies, whatever judgment 

 we may form as to its universal application, 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 immense 

 variety of compound tints, until at length it becomes so thin 

 (as in the case of the soap-bubble) as to be incapable of re- 

 flecting any colour at all : the last colour it reflects being- 

 orange, yellowish -white, and finally blue, before they vanish ; 

 these are called colours of the first order. Now, on this sub- 

 ject, Newton says, <' The blue of the first order, though very 

 faint and little, may possibly be the colour of some substances ; 

 and particularly the azure colour of the sky seems to be of this 

 order. For all vapours, when they begin to condense and 

 coagulate into small parcels, become first of that bigness 

 whereby such an azure must be reflected before they can con- 

 stitute clouds of other colours. And so this being the first 

 colour which vapours begin to reflect, it ought to be the 

 colour of the finest and most transparent skies in which 

 vapours are not arrived to that grossness requisite to reflect 

 other colours, as we find it by experience*." In another pro- 

 position, he says : " If we consider the various phaenomena of 

 the atmosphere, we may observe, that when vapours are first 

 raised, they hinder not the transparency of the air, being di- 

 vided into parts too small to cause any reflection in their su- 

 perficies. But when, in order to compose drops of rain, they 

 begin to coalesce and constitute globules of all intermediate 

 sizes, those globules, when they become of a convenient size 

 to reflect some colours, and transmit others, may constitute 

 clouds of various colours, according to their sizes; and I see 

 not what can be rationally conceived in so transparent a sub- 

 stance as water for the production of these colours, besides 

 the various sizes of its fluid and globular parcels f." 



The theory of Newton, therefore, embraces the colour of 

 clouds, whether by reflected or transmitted light, as well as 

 that of the blue sky. He applied a modification of the same 

 theory to explain the corona: round the sun and moon 1. The 



• Optics, Book ii. Part iii. Prop. vii. \ Ibid. Prop. v. cml. 



X Book ii. Part iv. Obs. Vi. 



