COLOUR. 



331 



simple experiment. Place a small quantity of strong 

 volution of gum Arabic between two plates of mo- 

 ther of pearl, which, when hard, must be separated 

 from the plates ; it will then be found to exhibit, either 

 by reflection or refraction, all the fine colours of the 

 real shell. The blue colour of the sky is to be ac- 

 counted for on the principle that the air reflects the 

 blue part of the rays only, and the diversified ap- 

 pearances of the clouds under different circumstances 

 arise from differences in their density or position. 

 The immense quantity of vapour between a spectator 

 on the earth, and the sun, presents an obstruction to 

 the easy passage of light, so that only the more 

 dense rays can reach the eye, the weaker rays or 

 light of smaller particles being retarded ; hence the 

 morning and evening skies present those beautiful 

 assemblages of yellow, orange, and red clouds. So 

 it is in looking along a street at night ; the lamps 

 change their colour as they recede from us, becoming 

 more and more inclined to red, according to their 

 distances ; and divers have always remarked the red 

 appearance of objects under water. 



In the morning and evening, when the sun is near 

 the horizon, the rays have to remove a great quantity 

 of dense vapour before they reach the eye ; and 

 the less ponderable rays will be obstructed, the 

 more ponderable or red rays being the only ones 

 which reach the spectator ; hence the red appear- 

 ance of the sun in the morning and evening. 



Although opticians, in considering the subject of 

 colours, usually regard the seven of the prismatic 

 spectrum as simple and distinct from each other, yet, 

 on a close examination of the image formed by the 

 prism, it will be found that the colours pass into 

 each other by degrees, and that therefore there is a 

 great diversity ot hue even in the space usually al- 

 lotted to one colour. Some hold that there are only 

 three primary colours; red, blue, and yellow, be- 

 cause all the others can be compounded of these. 

 This view of the subject is rendered highly probable, 

 by the fact that the colours, said to be compound, i. e. 

 orange, green, indigo, and violet, are intermediate be- 

 tween the primary, red, yellow, and blue, as exhibited 

 in the prismatic spectrum. The shade and tint of the 

 compound colours may be greatly varied by mixing the 

 simple colours which form them in different propor- 

 tions. Thus green, which is a compound of blue and 

 yellow, may be made of very various tints, by mixing 

 these colours in different proportions, and still the 

 resulting colour will be called green ; but the shade 

 of either yellow or blue can only be altered by add- 

 iii- white ; for should any other colour be added to 

 the yellow or blue, the compound can no longer 

 receive the name of yellow or blue. Much practice 

 is necessary to enable us to distinguish slight differ- 

 ences in the shades and tints of colours, and some 

 eyes are totally incapacitated for this, especially 

 where particular colours are concerned. Many in- 

 stances might be brought forward of this nature ; 

 we need only mention Dugald Stewart, Dalton, and 

 Troughton, who could not distinguish certain colours 

 from others. But in all cases where this defect exists, 

 it has been found that the persons are insensible to 

 red light and all the compound colours into which 

 it enters. 



A very simple experiment of M. Buffon's has 

 opened a new field in the science of colours, which 

 has been productive of results of great utility in a 

 practical point of view. Buffon having placed a red 

 wafer on a sheet of white paper, and fixed his eye on 

 Jt steadily for a few seconds, then having looked to 

 another part of the paper, he saw a green spectrum 

 of ihe same size as the wafer. By using wafers of 

 different colours, differently coloured spectra were 

 produced, as follows : 



Colour of the H'afer. 

 Black, 

 White, . 

 Red, . 

 Orange, . 

 Yellow, . 

 Green, . 

 Blue, . 

 Indigo, . 

 Violet, 



Colour of the Spectrum. 



White. 



Black. 



Bluish green. 



Blue. 



Indigo. 



Violet, with a little red. 



Orange red. 



Orange yellow. 



Bluish green. 



These spectra are called accidental or contrasting 

 colours ; they are rendered still more distinct, if the 

 wafers be viewed upon a black ground, and the eye 

 transferred from that to a white ground. The acci- 

 dental colours may all be found in another way. 

 Take the circle alluded to before, having the prismatic 

 colours painted on its surface, in the proportion as 

 marked in the figure below. If this circle be whirled 

 round its centre, it will, as observed above, appear 

 as a white disc. But suppose any one of the colours 

 taken out, or painted black, then will the circle, 

 when whirled round, exhibit another colour, which 

 shall be the accidental of that which has been black- 

 ened or omitted. Thus, if the red be left out, the 

 disc, when whirled, will appear green, and by leav- 

 ing out the other colours in succession their respec- 

 tive accidentals may be shown. The accidental 

 colour of any of the colours in the circle may be 

 easily found. Thus, 



If we omit violet, wishing to find its accidental 

 colour, then the remaining arch will be A E B, 

 whose centre of gravity is the point m m, the green 

 arch, but not in the centre of that arch, 1 eing some- 

 what towards the yellow ; therefore the contrasting 

 colour of violet will be green, mixed with a little 

 yellow. Hence, by drawing a diameter through the 

 centre of the omitted arch, the other extremity of 

 this diameter will be in the centre of the accidental 

 colour, and so of the rest. The explanation of the 

 phenomena of accidental colours is at once simple and 

 satisfactory. When the eye is directed for some 

 time, to a red spot for instance, the retina becomes 

 strongly excited by the red rays, and its sensibility 

 to weaker impressions of rays of that colour must, 

 therefore, be for a time destroyed ; just as the 

 palate, when accustomed to a particular taste, ceases 

 to feel its impression when weak. Now white is a 

 compound of all the colours, and, therefore, when 

 the eye is turned from the red wafer to the white 

 ground, it will necessarily become insensible to the 

 red rays which enter into the combination of the 

 white; the eye, therefore, receives an impression 

 of all the others combined, which, we have seen, 

 must be green, the accidental colour of the red. It 

 deserves to be noticed, that none of the three simple 

 colours, before alluded to, ever appear as the at:ci- 

 dental of a compound, but each contrasting colour 

 of a simple one is compounded of the other two ; 

 thus, the accidental colour of red, i. e. green, is a 

 compound of blue and yellow 



