Dec. 1, 1885.] 



♦ KNOWLEDGE ♦ 



51 



light reflected from tlic bottom. It must be noted that 

 ihe suspension of foreign bodies in water materially 

 affects its colour, since certain vegetable matter 

 converts the normal blue into a very greenish blue ; and 

 wo have ourselves seen water brilliantly green, as in a 

 dyer's vat, from the presence of enormous numbers of the 

 Euglcna viridit. Decaying organic matter tinges water 

 brown, as may be well seen in some of the torrents in 

 the rivers in North Wales; but these are all departures 

 from what may be called the natural colour exhibited by 

 wr.ter in great thickness, the blue of which we have 

 r.lready spoken. The green of green leaves presents some 

 curious chromxtic anomalies, inasmuch as, with a large 

 proportion of green and yellow, it also contains a little 

 blue and violet and a good deal of crimson ! This has 

 b^en determined by the aid of that instrument known 

 rs the spectroscope, a description of which is promised 

 by the Editor of this joiirn.il in a forthcoming series of 

 papers on " Light-sifting." Those especially interested 

 in this branch of the subject, then, must, pro tempore, 

 take our word for the results thus obtained until the time 

 arrives for an account of the instrumental means by the 

 aid of which they have been arrived at. We may, how- 

 ever, here once more reiterate in a succinct form^vhat 

 we have been endeavouring to explain in some detail, 

 viz., that the colour.? nf objects in the natural world, as 

 we see them, have their origin in the selective action of 

 those objects on white light, certain constituent vibra- 

 tions of which are absorbed and converted into heat, 

 while the remaining ones (mingled with more or less pure 

 white light superficially reflected) impress the eye with 

 ihe colour corresjwnding with their length. 



Thus much for the ordinary production of colour. It 

 remains to say something of its abnormal, unusual, or 

 extraordinary production. Bearing in mind what we 

 hive previously said (p. 16) about colour, as such, exist- 

 ing only in the sensorium, it is not difficult to conceive 

 that if we can contrive in any way to cause a similar 

 excitation in that part of the brain concerned in the 

 production of the sensation, we shall experience it, albeit 

 we are in total darkness. Thus Helmholtz tells us that 

 an electrical current passing thi-ough the forehead to the 

 hand, causes a seemingly wild rush of colours, although 

 no light whatever may be present. Every one, too, 

 coming suddenly out of sun.shine into a dimly-lighted 

 church, and kneeling with his head in his hands, must 

 be familiar with the brilliant spectra that pass before his 

 closed eyes. Of complementary spectra we shall have 

 something to say in a future paper. Even a black spiral 

 on a white disc set in revolution in an apparatus to be 

 hereafter described, will, if rotated at a pi-oper speed, 

 seem greenish, and when turned more quickly, reddish ; 

 while santonin, taken internally, causes everything white 

 to appear greenish-yellow. 



And this brings us to the very remarkable subject of 

 colour-blindness, or total incapacity for seeing certain 

 colours, with which many people are quite unwittingly 

 afflicted. Daltoii, the famous chemist, was thus affected. 

 Red and green were ab.solutely undistinguishable by 

 him ; and as he was the first to give a scientific descrip- 

 tion of this strange infirmity, it is still commonly called 

 Daltonism. So comparatively common is this pai'ticular 

 defect that it has been calculated that between five and 

 six per cent, of people in this country are more or less 

 afBicted with it. We have our.selves known more than 

 one person who could detect no difference whatever in 

 the colour of the berries of a holly branch and its leaves, 

 and only recognised them by their shape. Pole and 

 Mivart, among other eminent men of science, are sufferers 



from this strange defect. The serious importance of its 

 existence will be recognised when we consider how many 

 thousands of people are daily employed on the railways 

 of the United Kingdom, where the conventional signals 

 indicating caution and danger are green and red respec- 

 tively, the two very colours which more than a twentieth 

 of the entire population are unable to distinguish one 

 from another. Happily this has attracted serious attention 

 everywhere, and we believe that some kind of colour-te.st 

 is now applied to all signalmen, guards, engine-drivers, 

 and others, whose failure to discriminate between these 

 two colours may be fraught with the most frightful 

 consequences. In 1870, Professor Holmgren found among 

 266 employes of the Upsala-Gefle Railway in Sweden 

 that thirteen were colour-blind. The test ordinarily 

 employed consists of a series of skeins of coloured wool, 

 which the person \inder examination is required to match. 

 Seebeck succeeded in making himself temporarily colour- 

 blind by wearing ruby-red spectacles during the greater 

 part of one day. When they were removed he could only 

 discern two colours. A rarer form of colour-blindness 

 exists in which the sufferer sees only (what he calls) red 

 and blue — including yellow in the former designation. 

 Finallj', in some rarer cases still, patients have been fou.nd 

 practically destitute of any sense of colour at all ; in fact, 

 who saw the whole face of nature in mere light and shade, 

 like a mezzotint engraving, a condition of things which 

 may be imitated by the familiar Christmas experiment 

 of salting a spirit-lamp and using the flame thus charged 

 with sodium vapour as the sole source of illumination of 

 a room. How under such illumination all traces of colour 

 vanish and the spectators' faces assume a cadaverous and 

 ghastly hue, very many of our readers know. Something 

 of the sort is seen with the light of the ordinary "snap- 

 di-agon " of this season of the year ; though the absence 

 of salt from the burning brandy somewhat mars the 

 effect. The physiological causes of colour-blindness are 

 somewhat obscure. It probably has its origin in some 

 actual defect in those parts of the retina which are par- 

 ticularly set in vibration by the red rays. This, however, 

 is a question which we do not pi-opose to pursue here. It 

 is enough if we have conveyed an intelligible idea of the 

 phenomenon itself. 



THE SOCIAL WASPS, 

 Bv E. A. Butler. 



iff»| L^ fwiF the Vespidce, or social wasps, we have 

 V^if^^ seven British species, including the 

 hornet, which is by far the largest and 

 most easily recognisable. The discrimina- 

 tion of the other species is not by any 

 means an easy matter, and needs a very 

 clo.se attention to minute details. But the 



hornet is at once distinguished both by its size and 

 colour ; its hues are brown and yellow, instead of black 

 and yellow, as is the case with all the other species. 



Before, however, we can properly understand the 

 differences of the species, or the reasons for their 

 invasion of our homes and their pilfering of our food, 

 it will be needful to sketch the life-history of a Ves2}a. 

 As the plan is nearly the same in all, we need not at 

 present particularise species, but only premise that we 

 choose a subterranean, as being more common than an 

 arboreal, builder. 



We begin with a fine old female, or queen, as she is 

 called, whom the warmth of advancing spring has ai'oused 

 from her long winter sleep. She is an ancient dame, 



