l^S ALTERATIONS OF THE SOLAR LIGHT. 



others in which it appears yellow, orange coloured, or red. 

 According to Mr. Hassenfratz these different effects depend 

 in general on the state of the atmosphere, the difference of 

 the latitude, and the elevation above the sea. As to the ul- 

 timate cause of these phenomena, it was natural to ascribe 

 them, as Mr. Hassenfratz does, to the suppression of a part 

 of the rays of the solar light in its passage through the at- 

 mosphere. Newton has already announced this property of 

 transparent mediums, to stop certain of the rays that enter 

 them, letting the rest pass on ; and this celebrated philoso- 

 pher even remarks, that they are frequently absorbed one 

 after another at different distances from the surface at which 

 the light entered ; and he quotes for example the various 

 tints exhibited in succession by a coloured fluid in a conical 

 glass, which is placed between the eye and the light, and 

 raised so as to have the thickness traversed by the visual ray 

 continually increasing. 

 Object of the Now Mr. Hassenfratz proposes to determine the number 



author to deter- and kinds of rays, the suppression of which occasions the 

 mine \he kind _ \ J ' , rr . . . . . . . . 



and quantity of various tints, that alter the primitive whiteness or the solar 

 rays intercept- ijght, The means he has employed are founded on a rule 

 given by Newton, to determine the colour produced by a 

 given mixture of rays of different kinds taken from those 

 that compose the solar spectrum. It follows from this, that, 

 if we can know the sorts of rays that the atmosphere takes 

 away from the solar light, we shall know by necessary con- 

 sequence the colour produced by the mixture of the species 

 remaining, and we may judge whether this colour be the 

 same as that, under which the disk of the sun presents it- 

 self. Here we must observe, that the mixture producing a 

 given colour may be more or less compounded, because a 

 colour does not change, at least with regard to its species, 

 by the addition of parts of the spectrum situate on each 

 side at equal distances from the point considered as the cen- 

 tre of this colour. For instance, if we add to the green its 

 two contiguous colours, blue and yellow, we still have green ; 

 and the mixture will remain green, if we farther add indigo 

 and orange, one of which is contiguous to the blue and the 

 other to the yellow. Nothing but direct experiment there- 

 fore can indicate the species of rays absorbed in their pas- 



