AND THE MORNING AND EVENING RED. 329 



This result must also find its application in the atmosphere, 

 where vesicles of vapour take the place of the thin plates. Those 

 vesicles which float in the air in serene weather must cer- 

 tainly be very thin ; and if we assume that their thickness does 

 not exceed the fourth part of an undulation of the violet light, 

 the blue colour of the reflected light of the firmament would 

 follow as a necessary consequence, and the clearer the air, that 

 is, the finer the vesicles, the more deeply blue would it appear. 



When, on the other hand, the air becomes more moist and 

 the vesicles no longer possess the requisite fineness, it might be 

 imagined that the firmament, instead of blue, ought to exhibit 

 the other colours in their proper order. This, however, is not 

 the case. When in moist weather the vesicles increase in 

 thickness, new fine vesicles are in the act of forming at the 

 same time, so that we never have a definite thickness for all, 

 but at most a limit value, which does not exclude the smaller 

 ones. When this limit is a little exceeded, then with a large 

 quantity of blue we have a little white. As the thickness in- 

 creases we have blue, white, yellowish-white, and again blue, 

 white, yellowish-white, orange, &c. Hence new colours per- 

 petually add themselves to those already present, and the mix- 

 ture of all can only add to the original blue a more or less 

 perfect white, by which the former obtains a milky appearance, 

 and may slowly pass into white altogether. 



With this the actual process in the atmosphere coincides 

 most completely. Even in clear weather, the sky towards the 

 horizon generally appears whitish, for the eye thus directed, 

 besides looking through a thicker mass of air, looks along the 

 surface of the earth, where, at least at certain moist places, pro- 

 bably thicker vesicles are suspended than at greater elevations. 

 As the air becomes moister the white expands and the whole 

 appearance of the firmament becomes duller. In fogs and 

 clouds the thickness of the bladders must be assumed to be 

 much greater, but we must not therefore expect, as Newton did, 

 that the clouds can produce, by reflexion, definite colours of a 

 higher order, for it would be quite unnatural to suppose that 

 the cloud is composed of vesicles all of the same thickness. On 

 the contrary, we must expect here the greatest variety, so that 



SCI EN. MEM.— iVa/. Phil. Vol. I. Part IV. 2 A 



