4IO 



NATURE 



[Sept. I, 1 88 1 



still unconvinced, he unj, I believe, almost alone in his prefer- 

 ence for the emision theory. The phenomena of interference, 

 in fact, left hardly any — if any — room fur doubt, and the subject 

 was finally set at rest by Foucault's celebrated experiments in 

 1850. Accrjrdiiig to the uudulatory theory the velocity of Ui;ht 

 ought to be greater in air than in water, while if the emission 

 theory were correct the reverse would be the case. The velocity 

 of li;iht — iS6,ooo miles in a second— is, however, so great that, 

 to determine its rate in air, as compared with that in water, 

 might seem almost hopeless. The velocity in air was, neverthe- 

 less, determined by Fizeau in 1849, W means of a rapidly 

 revolving wheel. In the following year Foucault, by means of 

 a revolving mirror, de no istrated that the velocity of light is 

 gi-eater in air tlian in water — thus completing the evidence in 

 favour of the undulatory theory of li^ht. 



The idea is now gaining ground, that, as maintained by 

 Clerk-Maxwell, light itself is an electro-magnetic disturbance, 

 the luminiferous ether being the vehicle of both light and 

 electricity. 



Wiinsch, as long ago a; 1792, had clearly shown that the three 

 primary colours were red, green, and violet ; but his results 

 attracted little notice, and the general view used to be that there 

 were seven principal colours — red, orange, yellow, green, blue, 

 indigo, and violet ; four of which — namely orange, green, indigo, 

 and vi ilet — were considered to ari,e from mixtures of the other 

 three. Red, yellow, and blue were therefore called the primary 

 colours, and it was supposed that in order to produce white light 

 these three colours mu^t always be present. 



Helmholtz, hosvever, again showed, in 1852, that a colour to our 

 unaided eyes identical with white, was produced by combining 

 yellow with indigo. At that time yellow was considered to be a 

 simple colour, and this, therefore, was regarded as an exception 

 ■to the general rule, that a combination of three simple colours is 

 required to produce whi^e. Again, it was, and indeed still is, the 

 general impression that a combination of blue and yellow makes 

 green. This, however, is entirely a mist,ake. Of course we all 

 know that yellow paint and blue paint make green paint ; but 

 this re-ults from absorption of light by the semi-transparent 'olid 

 particles of the pigments, and is not a mere mixture of thecolnurs 

 proceeding unaltered f ro n the yellow and the blue particles ; 

 moreover, as can easily be shown by two sheets of coloured paper 

 and a piece of window glass, blue and yello v light, when com- 

 bined, do not give a trace of green, but if yjure would produce 

 the effect of white. Green, therefore, is after .all not produced 

 by a mixture of blue and yellow. On the other hand Clerk- 

 Maxwell proved in 1S60 that yellow could be produced by a 

 mixture of red and green, which put an end to the pretensim of 

 yellow to be considered a primary element of colour. From these 

 and other cou.sidera'.ions it would seem, therefore, that the three 

 primary colour^ — if such an expressi n be retained — are red, 

 green, and violet. 



The existence of rays beyond the violet, though almost in- 

 visi'ile to our eyes, had long been demonstrated by their chemical 

 action. Stokes, ho.'. ever, showed in 1S52 that their existence 

 might be proved in another manner, fo.' that there are certain sub- 

 stances which, when excited by them, emit light visible to our 

 eyes. To this phenomenon he gave the name of fluorescence. 

 At the other end of the spectrum Abney has recently succeeded 

 in photographing a large number of lines in the infra-red portion, 

 the existence of which was first proved by Sir William 

 Herschel. 



From the rarily, and in many cases the entire absence, of 

 reference to blue, in ancient literature, Geiger — adopting and 

 extending a su_'gesli m first thrown out by Mr. Gladstone — has 

 maintained that, even as recently as the time of Homer, our 

 ancestors were blue blind. Though for my part I am unable to 

 adopt this view, it is certainly very remarkable that neither the 

 Rigveda, which consists alinost entirely of hymns to heaven, 

 nor the Zendave ta, the Biljle of the Parsees or fire-worshippers, 

 nor the Old Testament, nor the Homeric poems, ever allude to 

 the sky as blue. 



On the other hand, from the d i« n of poetry, the splendours of 

 the morning and evening skies have excited the admiration of 

 mankind. As Ruskin says, in language almost as brilliant as 

 the .sky itself, the whole heaven, "from the zenith to: he horizon, 

 becomes one molten, mantling sea of colour and fire ; eveiy 

 black bar turns into ma-sy gold, every rij'ple and wave into un- 

 sullied shadowlesi crim on, and purple, and scarlet, and cohnirs 

 for which there are no words in language, and no ideas in the mind 

 — things which can only be conceived while they are visible ; 



the inten e hollow blue of the upper sky melting through it all, 

 showing here deep, and pure, and lightness ; there, modulated by 

 the filmy, formless body of the transparent vapour, till it is lost 

 imperceptibly in its crimson and g old." 



But what is the explanation of these gorgeous colours ? why is 

 the sky blue ? an^ why are the sunrise and sunset crunson and 

 gold? It may be said that the air is blue, but if so how can 

 the clouds assume their varied tints? Briicke showed that very 

 minute particles suspended in water are blue by reflected light. 

 Tyndall has taught us that the blue of the sky is due to the re- 

 flection of the blue rays by the minute particles floating in the 

 atmosphere. Now if from the white light of the sun the blue 

 rays are thus selected, those which are transmitted will be 

 yellow, orange, and red. Where the distance is short the trans- 

 mitted light v\ ill appear yellowish. But as the sun sinks to- 

 wards the horizon the atmospheric distance increases, and conse- 

 quently the number of the scattering p.articles. They weaken in 

 succession the violet, the indigo, the blue, and even disturb the 

 proportions of green. The transmitted light under such cir- 

 cumstances must pass from yellow through orange to red, and 

 thus, while we at noon are admiring the deep blue of the sky, 

 the same rays, robbed of their blue, are elsewhere lighting up the 

 evening sky with all the glories of sunset. 



Another remarkable triumph of the last half-century has been 

 the discovery of photography. At the commencement of the 

 century Wedgwood and Davy observed the effect produced by 

 throwing the images of objects on paper or leather prepared 

 with nitrate of silver, but no means were known by which such 

 images could be fixed. This was first effected by Niepce, but 

 his processes were open to objections which prevented them 

 from coming into general u-e, and it was not till 1839 that 

 Daguerre invented the process which was justly named after 

 him. Very soon a further improvement was effected by our 

 countryman Talbot. He not only fixed his " T.albotypes " on 

 paper — m itself a great convenience — but, by obtaining a ne- 

 gative, rendered it possible to take off any number of positive, 

 or natural, copies from one original picture. 



We owe to Wheatstine the conception that the idea of 

 solidity is derived from the combination of t\\ o pictures of the 

 same object in slightly different perspective. This he proved in 

 1833 by drawing two outline, of some geometrical figure or 

 other sim]ile object, as they would appear to either eye respec- 

 tively, and then placing them so that they might be seen, one by 

 each eye. The "stereo; cope," thus produced, hai been greatly 

 popularised by photography. 



For 2000 years the art of lighting had made little if any 

 progre.ss. Until the close of the last century, for instance, our 

 bghthouses contained mere fires of wood or coal, though the 

 construction had vastly improved. The Eddystone lighthouse, 

 for instance, was built by Smeaton in 1759 ; but for forty years 

 it, light consisted in a ro-,v of taUow candles studs in a hoop. 

 The Argand lamp was the first great improvement, followed by 

 gas, and in 1863 by the electric light. 



Ju^t as light was long supposed to be due to the emission of 

 material particles, so heat was regarded as a material, though 

 ethereal, substance, which was added to bodies when their tem- 

 perature was raised. 



Pavy's celebrated experiment of melting two pieces of ice by 

 rubbing them again t one another in the exhausted receiver of 

 an air-pump had convinced him that the cause of heat was the 

 motion of the invisible particles of bodies, as had been long 

 before suggested by Newto;-, Boyle, and Hooke. Rumford and 

 Young also advocated the same view. Nevertheless, the general 

 opinion, even until the middle of the present century, was that 

 heat was due to the presence of a subtle fluid known as " caloric," 

 a theory which is now entirely abandoned. 



'Ihe determination of the mechanical equivalent of heat is 

 mainly due to the researches of Mayer and Joule. Mayer, in 

 1S42, pointed out the mechanical equivalent of heat as a funda- 

 mental datum to be determined by experiment. Taking the 

 heat produced by the condensation of air as the equivalent of 

 the work done in compressing the air, he obtained a numerical 

 value of the mechanical equivalent of heat. There was, how- 

 ever, in these experiments, one weak point. The matter 

 operated on did not go through a cycle of changes. He 

 assumed that the production of heat was the only eflect of the 

 work done in compressing the air. Joule had the merit of being 

 the first to meet this possible source of error. He ascertained 

 that a weight of i lb. would have to fall 772 feet in order to 

 raise the temperature of i lb. of water by 1° Fahr. Hirn 



