JrxE 1, 1886.] 



♦ KNOWLEDGK ♦ 



237 



Since everything is black in the dark, and moreover has no 

 colour in itself, it follows that colour is in some way a pro- 

 perty of light. Now light, which is itself invisible, is due 

 to vibrations or oscillations set up in all directions by any 

 luminous body — whether the sun or a rushlight — in the 

 ethereal medium which pervades all space, and is composed 

 of rays of different refrangibilities — i.e. change of direction 

 in passing from one medium into another. White light is 

 due to the combination of all the.se rays, ranging through 

 innumerable gradations of colour from red to violet, and it 

 is to the absence of one or more of them that the infinite 

 variety of colours is due. If a body is quite opaque, or 

 otherwise so constituted as to absorb none of the rays, it 

 ajipears white ; if it absorbs them all it appears black ; 

 if it absorbs green, blue and violet, and not red, it ap- 

 pears red ; if it absorbs red, orange, and violet, and 

 returns or reflects green, it appears green. The colours 

 which bodies reflect is therefore regulated by their structure ; 

 the way in which their molecules are arranged determines 

 the number and character of the light vibrations or ether 

 waves which are returned to the eye and which rule the 

 colour we see — e.g. charcoal and the diamond ai-e both pure 

 airbou ; the dull opacity of the one and the trembling 

 splendour of the other are solel}' due to the arrangement of 

 the several molecules of each. 



It is thus obvious that any change in the nature or 

 structure of a thing is accompanied by change in its colour, 

 and to this cause the various pigments in plants is to be 

 referred. 



All growth involves expenditure of the energy -which the 

 jilant has stored within itself, and which becomes active 

 when the hydro-carbons combine with oxygen, resulting in 

 cellular change, and appearance of other colours than the 

 green, w-hich is due to chlorophyll. Thus may be explained 

 the colour of sprouting buds and young shoots and the more 

 or less intensitied colours of leaves and flowers — one and all 

 due to oxidation, the minutest changes inducing subtle 

 variations in colour. 



Whichever plants made most show of colour would the sooner 

 catch the eye of insects, however dim their perception of the 

 difference in colours might be, and would thus get fertilised 

 before plants which made less display. Thus have insects 

 been the main cause in the propagation of flowering plants ; 

 the plants in return developing the colour-sense in insects. 

 The flower nourish &s the insect ; the insect propagates the 

 flower. Other contrivances to meet the need for fertilisation 

 might be cited, as the markings upon the )ietalsto guide the 

 insect to the nectary ; the exhalation of scent by incon- 

 spicuous flowers, or by such as would attract visitors at 

 nigbt, and so forth ; but enough has been adduced to show 

 what is the chief, if not the sole, function discharged by 

 flowers — the attraction of insects to aid in securing cross- 

 fertilisation. Nor does the provision stop here. The fer- 

 tilised seed is not left to chance, but, like the fertilising pollen, 

 is entrusted to secondaiy agents, to the care of the birds 

 and the breezes. Where not scattered by the bursting of 

 the ovary it is winged with gossamer shafts, as in the dan- 

 delion, and carried by the wind, floated on gentlest zephyT 

 or rushing storm to a genial soil. Such wind-wafted seeds, 

 like wind-fertilised flowers, are rarely coloured ; neither are 

 the seeds of the larger trees, since their abundance ensures 

 notice by food-seeking animals ; nor the nuts, which are 

 protected by shelly coats. But other seeds enwrap them- 

 selves in sweet pulpy masses, called fruits, whose skins 

 brighten as they ripen, and attract the eye of fruit-loving 

 birds and beasts. The seeds pass through their stomachs 

 undigested, and are scattered by them in their flight over 

 wide areas. As with the brightest-hued and sweetest-scented 

 flowers, so it is with the brightest and juiciest fruits; they 



sooner attract the visitors whose services they need, and 

 thus gain advantage over less-favottred members of their 

 species, developing by the selective action of their devourers 

 into the finest and pulpiest kinds. 



THE VELOCITY OF LIGHT.* 



|N one of his early papers on " Optical Recrea- 

 tions" (volume v., p. 3.51), our contributor, 

 " A Fellow of the Royal Astronomical Society," 

 made cursory reference to the two methods 

 adopted for the determination of the rate at 

 which light travels, and expressed an opinion 

 that the mode devised by Foucault was un- 

 questionably the more trustworthy one. The fine volume 

 before us contains an elaborately' detailed description of the 

 most recent application of Foucault's apparatus to the deter- 

 mination of the velocity of light, by Professor Newcomb and 

 Professor Michelson in the United States, during the years 

 18S0, 1881, and 1882; and it has seemed to us that, without 

 encuml)ering our description with the minutire of optical and 

 mechanical ingenuity and of the various corrections applied 

 in reducing the observations, an account of the apparatus 

 employed and of the mode of employing it, by these fiimous 

 American observers, can scarcely fail to be instructive to all 

 interested in the most refined of all physical measurements. 



For the better apprehension of the nature of the work done 

 by the two Ameriatn professors, it will be better if, in the 



Fig. 1. 



outset, we endeavour to render the principle on which their 

 observations were made thoroughly intelligible. To this end 

 we propose to describe the form of apparatus first used by 

 Michelson, being a modification by him of that in which it was 

 originally devised by Foucault ; and we shall employ a dia- 

 grammatic Olustration instead of an actual section or eleva- 

 tion of the arrangement employed, our object being just now 

 to render the principle of the method intelligible. When we 

 come to treat of the shape which the apparatus as.sumed in the 

 hands of Newcomb, the reader will then be in a position to 

 appreciate the use and significance of the various details. 

 Turn we now to Fig. 1, where we arestipposed to be looking 

 down on to the top of Michelson's original form of apparatus. 

 S is a slit with its length square to the surface of this page. 

 Through it. reflected by a heliostat, passes the light from the 

 sun © , and falls upon a mirror R M, which turns round an 



* Measures of the Velocity of Light, made under the direction of 

 the Secretary of tlie Navy, during the years 1S80-S2, by Simon 

 Newcomb, Professor, U.S. Navy. Vol. II. parts 3 and i of the 

 Astronomical Papers prepared for the use of the American Epheme- 

 ris aitd Nautical Almanac: Washington, 1885. 



