109 



SCIENTIFIC SIDE-LIGHTS 



ation 





fresh generations of plants. ANDREW WIL- 

 SON Glimpses of Nature, ch. 22, p. 72. 

 (Hum., 1892.) 



538. COLOR OF PIGMENTS Absorp- 

 tion and Reflection Unite to Determine 

 The Rose Seen by Light Reflected Back 

 through Its Substance Exhaustion by 

 Waste of Echoes. Pigments are composed 

 of particles mixed with a vehicle; but how 

 intimately soever the particles may be 

 blended, they still remain particles, sepa- 

 rated it may be by exceedingly minute dis- 

 tances, but still separated. To use the 

 scientific phrase, they are not optically con- 

 tinuous. Now, wherever optical continuity 

 is ruptured we have reflection of the incident 

 light. It is the multitude of reflections at 

 the limiting surfaces of the particles that 

 prevents light from passing through glass j 

 or rock salt., when these transparent sub- 

 stances are pounded into powder. The light 

 here is exhausted in a waste of echoes, not 

 extinguished by true absorption. It is the 

 same kind of reflection that renders the 

 thunder-cloud so impervious to light. Such 

 a cloud is composed of particles of water 

 mixed with particles of air, both separately 

 transparent, but practically opaque when 

 thus mixed together. In the case of pig- 

 ments, then, the light is reflected at the lim- 

 iting surfaces of the particles, but it is in 

 part absorbed within the particles. The re- 

 flection is necessary to send the light back 

 to the eye; the absorption is necessary to 

 give the body its color. The same remarks 

 apply to flowers. The rose is red in virtue, 

 not of the light reflected from its surface, 

 but of light which has entered its substance, 

 which has been reflected from surfaces 

 within, and which in returning through the 

 substance has had its green extinguished. 

 A similar process in the case of hard green 

 leaves extinguishes the red, and sends green 

 light from the body of the leaves to the eye. 

 TYNDALL Lectures on Light, lect. 1, p. 34. 

 (A., 1898.) 



539. COLOR OF THE SKY Hues of 



Flowers Due to Absorption Fine Particles 

 Make Blue of Sky Alpine Sunrise and Sun- 

 set. First, then, with regard to the sky; 

 how is it produced, and can we not repro- 

 duce it? Its color has not the same origin 

 as that of ordinary coloring matter, in 

 which certain portions of the white solar 

 light are absorbed, the color of the body be- 

 ing that of the light which remains. A vio- 

 let is blue because its molecular texture 

 enables it to quench the yellow and red con- 

 stituents of white light, and to send back 

 the blue from its interior. A geranium is 

 red because its molecular texture is such as 

 quenches all rays except the red. Such col- 

 ors are called colors of absorption; but the 

 hue of the sky is not of this character. The 

 blue light of the sky is scattered light ; and, 

 were there nothing in our atmosphere com- 

 petent to scatter the solar rays, we should 

 see no blue firmament, but the mere dark- 



ness of infinite space. The blue of the sky 

 is produced by perfectly colorless particles. 

 Smallness of size alone is requisite to insure 

 the selection and reflection of this color. Of 

 all the visual waves emitted by the sun, the 

 shortest and smallest are those correspond- 

 ing to the color blue. To such small waves 

 minute particles offer more obstruction than 

 to large ones, hence the predominance of 

 blue color in all light reflected from such 

 particles. The crimson glow of the evening 

 and the morning, seen so finely in the Alps, 

 is due, on the other hand, to transmitted 

 light; that is to say, to light which, in its 

 passage through great atmospheric dis- 

 tances, has its blue constituents sifted out 

 of it by repeated collision with suspended 

 particles. TYNDALL Heat a Mode of Motion, 

 lect. 16, p. 484. (A., 1900.) 



540. 



May Be Produced 



Artificially Light Liberates Atoms from 

 Vapor. We can liberate, in air, particles of 

 a size capable of producing a blue as deep 

 and pure as the azure of "the firmament. In 

 fact, artificial skies may be thus generated, 

 which prove their brotherhood with the nat- 

 ural sky by exhibiting all its phenomena. 

 There are certain chemical compounds 

 aggregates of molecules the constituent 

 atoms of which are readily shaken asunder 

 by the impact of special waves of light. 

 Probably, if not certainly, the atoms and 

 the waves are so related to each other, as 

 regards vibrating period, that the wave- 

 motion can accumulate until it becomes dis- 

 ruptive. A great number of substances 

 might be mentioned whose vapors, when 

 mixed with air and subjected to the action 

 of a solar or an electric beam, are thus de- 

 composed, the products of decomposition 

 hanging as liquid or solid particles in the 

 beam which generates them. . . . Like 

 the natural sky, the artificial one shows all 

 the colors of the spectrum, but blue in ex- 

 cess. TYNDALL Fragments of Science, vol. i, 

 ch. 5, p. 137. (A., 1897.) 



541. COLORATION, PROTECTIVE, UNI- 

 VERSAL Natural Objects of Every Kind Imi- 

 tated by Living Beings. Protective colora- 

 tion, in some of its varied forms, has not 

 improbably modified the appearance of one- 

 half of the animals living on the globe. The 

 white of arctic animals, the yellowish tints 

 of the desert forms, the du*sky hues of cre- 

 puscular and nocturnal species, the trans- 

 parent or bluish tints of oceanic creatures, 

 represent a vast host in themselves ; but we 

 have an equally numerous body whose tints 

 are adapted to tropical foliage, to the bark 

 of trees, or to the soil or dead leaves on or 

 among which they habitually live. Then we 

 have the innumerable special adaptations to 

 the tints and forms of leaves, or twigs, or 

 flowers ; to bark or moss ; to rock or pebble ; 

 by which such vast numbers of the insect 

 tribes obtain protection; and these various 

 forms of coloration are equally prevalent in 



