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♦ KNOWLEDGE ♦ 



(May 25, 1883. 



stages in the selective process are quite as essential to 

 Darwin's argument as the highly-evolved display of the 

 peacock, the turkey gobbler, and the Argus pheasant The 

 one set of exiuii[.li-s shows us the mechanism actually at 

 ■work ; the other set shows us the developed result. 



The blackcap conies over to us about the middle of April, 

 and continui'S in full song till the end of June. Being a 

 conspicuous and brisk little bird — far more likely to invite 

 a stray stout' from the murderous hand of ingenuous youth 

 than the dull-hued sparrow, or even the nimble challinch — 

 it hides its black-topped liead for the most part under the 

 shade of thickets ; but it is very fond of gardens, where 

 not too e.xposed, as it is a confirmed fruit-eater, given to 

 making great depredations among the raspberries and 

 currants. It is a noteworthy fact, however, that the young 

 blackcaps will eat nothing but insects ; it is not till they 

 have attained to years of discretion that they take to their 

 hnal habit of plundering fruit-bushes. This trait, common 

 to many, if not to most, fruit-eaters, maj' be put side by 

 side with the one noted by Mr. A. E.. Wallace, that the 

 young humming-birds, which are developed flower-haunting 

 swifts, will eat nothing but spiders and small flies. In 

 both cases, the facts point back curiously to the 

 original habits of the whole race. There can be 

 very little doubt that all birds were at first carnivorous, 

 piscivorous, or insectivorous, and the greater part of them 

 probably remain so to the present day. The practice of 

 eating grains and seeds came later ; while that of living 

 upon fruits, or the nectar of flowers, mu.st have been the 

 latest of all. Indeed, the development of succulent fruits 

 or berries seems to me to be a very recent acquisition on 

 the part of plants generally ; and it must have proceeded 

 side by side with the evolution of fruit-eating habits in the 

 correlated birds. Hence we lind the young still require to 

 be fed upon animal food ; and, indeed, the adult black- 

 caps, like many other similar mainly frugivorous species, 

 cannot get along for any length of time without a liberal 

 admixture of slugs and caterpillars in their food. On the 

 other hand, the most advanced fruit-eaters, such as the 

 parrots, readily reveit to carnivorous practices in confine- 

 ment ; and one New Zealand species, since the introduction 

 of sheep into the colony, has become a perfect pest to the 

 breeders by its partiality for animal dainties. ^ 



At the same time, adult blackcaps have now got so 

 thoroughly accustomed to their vegetable food that they 

 will not live in captivity without fresh fruits in summer, 

 and a little pear or apple in winter. Being such sweet 

 and varied singers, they arc often kept as " pets " by 

 people who " are fond of birds," much as one might keep a 

 great pianist in prison, by way of admiring his musical 

 genius. In their wild state, blackcaps love freedom and 

 retirement ; so that the confinement and publicity of a 

 cage must be perfect misery to such pert and agile, but 

 timid, birds. Thej' sing best in the month of ilay, when 

 food is most abundant, and when the nesting is going on. 

 Both birds take turn about in hatching the clouded, pale 

 brown eggs, one feeding while the other sits ; but they 

 don't like to be interfered with during the nesting period, 

 and will desert their eggs if any attempt is made to 

 intrude upon their privacy. All these little psychological 

 points have their own peculiar value in the economy of the 

 race ; and the more one studies birds, the more does one 

 begin to see that their dispositions are all as strictly corre- 

 lated as is outward structure to the general necessities of 

 the species. Blackcaps are above everything timid hangers- 

 on of civilisation ; and their habits are accommodated to 

 the needs of their situation. They cannot live readily in 

 very wild countries, and it is a matter of observation that 

 they have grown more and more numerous in Scotland 



with the spread of cultivation in the north, wliither they 

 have closely followed the currants and other garden fruits. 

 Indeed, the extent to which mankind lias modified and 

 altered the European fauna and ilora opens up a (juestion 

 still, I believe, only very partially understood. There 

 seems to be good reason for thinking that a very large 

 number of existing species can only have assumed their 

 present forms since the date when tillage first became- 

 common in the western Asian and Mediterranean regions. 



LAWS OF BRIGHTNESS.* 



By Richard A. Proctor. 



THERE are few subjects on which illusions are so widely 

 prevalent as that of brightness, whether the 

 brightness of self-lununous objects, or that of opaque 

 bodies under illumination. For example, nearly every one 

 who has not given the subject close attention supposes 

 that a luminous body, of either kind, shines with a 

 seemingly fainter light when at a distance than when near 

 at hand, independently of any absorptive effects exerted 

 by the medium through which the object is seen. Again, 

 it is quite a common error that a luminous sur- 

 face, when looked at slantwise, appears less luminous 

 than when looked at directly. When the elTects of tele- 

 scopic appliances are in question, mistakes of a yet more 

 widespread nature are met with. Thus, quite a large propor- 

 tion, even of those who are in the habit of using telescopes, 

 entertain the idea that by means of a telescope an object 

 may be made to appear brighter. The notion that the 

 moon, for instance, shines no more brightly as seen with a 

 large telescope than as seen with the naked eye is at a 

 first view scouted as ridiculous by many telescopists. 

 Again, those accustomed to the use of night-glasses are 

 usually prepared to asseverate that objects seen through 

 such glasses are rendered considerably brighter ; other- 

 wise, indeed, they argue a night-glass would be of no use 

 whatever. 



I propose here to consider a few of the leading facts 

 connected with the subject of brightness. They are of 

 extreme importance in many branches of research, and a 

 knowledge of them would prevent observers, as well as 

 theorists, from falling into many errors, either as to 

 methods, or as to the interpretation of observation. 



First, as to self-luminous objects. 



The first point to be noticed of self-luminous surfaces is 

 that they shine with equal brightness in whatever direc- 

 tion they may be viewed. If a red-hot poker be looked at 

 in a dark room it will be seen that the faces of the heated 

 square-sectioned part are of equal apparent brightness. The 

 experiment may equally well be tried in a lighted room ; 

 only, care must lie taken that the poker itself is not placed 

 in a strong light, for then, since a portion of its bright- 

 ness is reflected, there will be a certain degree of apparent 

 shading, the same in kind though not in degree, as though 

 the poker were an opaque body. A red-hot globe of iron 

 may be usefully studied in different lights, and especially 

 in full sunlight, when the apparent shading of the ball is 

 strikingly contrasted with the dead uniformity of the disc 

 presented when the ball is viewed in a dark room. 



The law here stated is strictly an experimental one. 

 There is nothing in the nature of things which should lead 

 us to expect this law. The law requires that the light 



* Tlieso papers, which are reprinted with little alteration front 

 the English Mechanic, in which they appeared several years since, 

 are introductory to a series of papers on " Spectroscopic Analysis/ 

 which will be commenced in Vol. V. 



