. ON PHYSICAL OPTICS, 345 



If we mix together, in proper proportions, any substances exhibiting 

 tliese colours in their greatest purity, and place the mixture in a light 

 sufficiently strong, we obtain the appearance of perfect whiteness ; but in 

 a fainter light the mixture is grey, or of that hue which arises from a com- 

 bination of white and black ; black bodies being such as reflect white light 

 but in a very scanty proportion. For the same reason, green and red sub- 

 stances mixed together usually make rather a brown than a yellow colour, 

 and many yellow colours, when laid on very thickly, or mixed with black, 

 become brown. The sensations of various kinds of light may also be com- 

 bined in a still more satisfactory manner, by painting the surface of a circle 

 with different colours, in any way that may be desired, and causing it to 

 revolve witi^such rapidity, that the whole may assume the appearance of a 

 single tint, or of a combination of tints, resulting from the mixture of the 

 colours. (Plate XXIX. Fig. 423... 426.) 



From three simple sensations, with their combinations, we obtain seven 

 primitive distinctions of colours ; but the different proportions in which 

 they may be combined, afford a variety of tints beyond all calculation. 

 The three simple sensations being red, green, and violet, the three binary 

 combinations are yellow, consisting of red and green ; crimson, of red and 

 violet ; and blue, of green and violet ; and the seventh in order is white 

 light, composed by all the three united. But the blue thus produced, by 

 combining the whole of the green and violet rays, is not the blue of the 

 spectrum, for four parts of green and one of violet make a blue, differing 

 very little from green ; while the blue of the spectrum appears to contain 

 as much violet as green : and it is for this reason that red and blue usually 

 make a purple, deriving its hue from the predominance of the violet. 



It would be possible to exhibit at once to the eye the combinations of any 

 three colours in all imaginable varieties. Two of them might be laid down 

 on a revolving surface, in the form of triangles, placed in opposite direc- 

 tions, and the third on projections perpendicular to the surface, which, 

 while the eye remained at rest in any one point, obliquely situated, would 

 exhibit more or less of their painted sides, as they passed through their 

 different angular positions ; and the only further alteration, that could be 

 produced in any of the tints, would be derived from the different degrees of 

 light only. The same effect may also be exhibited by mixing the colours 

 in different proportions, by means of the pencil, beginning from three 

 equidistant points as the centres of the respective colours. (Plate XXIX. 

 Fig. 427.) 



The ordinary atmospherical refraction cannot be determined in the 

 usual manner from the knowledge of its density, and of the angular direc- 

 tion of the incident or refracted light, since the constitution of the atmo- 

 sphere is such, that its density varies every where with its height, and the 

 curvature of the earth's surface causes the inclination of the strata through 

 which the ray passes to be perpetually changed ; the difference of tempera- 

 ture at different elevations increases also the difficulty of an exact calcula- 

 tion, and it is only very lately that Mr. Laplace,** by a comparison of 

 I astronomical with meteorological observations, has given a satisfactory 



* Mec. Cel. iv. 268. 



