September 1, 1896.] 



KNOWLEDGE. 



197 



brown gooney, and scientifically known as Diomedea 

 cliintnsis, is one of the commonest species of the genus. 

 Gould says of the birds of this species, in his "Birds of 

 Australia," that their actions and mode of flight differ 

 considerably from those of all other species of albatrosses, 

 their ai-rial evolations being far more easy, their flight 

 much higher, and their stoops more rapid. The cuneated 

 form of the tail, which is peculiar to the species, together 

 with the sUght and small legs and more delicate structure, 

 clearly indicate the most aerial species of its genus. 



On French Frigate Island, Laysan, and all other islands 

 visited by Palmer (Mr. llothschild's collector), the dark 

 albatross was fairly numerous. On Laysan Island the 

 breeding-place was to the south side of the island, where 

 the birds sat on the beach with their young, as is shown in 

 the photograph. 



The young feed by putting their beaks crosswise into 

 the old bird's mouth, and thus they catch the cast-up fish. 



Herr von Kittlitz, who visited these islands in 1828, 

 remarks that the sooty albatrosses are extremely foolish 

 and fearless. They can be caught with the hands, as they 

 must run a good distance before being able to get up on the 

 wing. If two birds meet, they bow to each other, uttering 

 a low cackling. When Herr Izenbeck, a friend of Herr von 

 Kittlitz, met one he used to bow to it, and the albatross 

 was polite enough to answer, bowing and cackling. This 

 could easily be regarded as a fairy tale ; but considering 

 that these birds, which did not even fly away when ap- 

 proached, had no reason to change then- customs, it seems 

 quite natural. 



The brown gooney in colouring is sooty brown above, 

 with the forehead a dirty white ; underneath the colours are 

 much paler and more grey. The quills and tail-feathers are 

 blackish brown, and the bill dark brown with a blackish 

 tip. The total length of the bird is thirty-three inches, 

 the wing being nineteen inches in length. 



The French Frigate Islands, Laysau, and other islands 

 in this group, are simply low sandbanks with little or no 

 vegetation on them, but they are covered with birds. One 

 would expect that these immense colonies would not 

 be altogether peaceable, and that such is the case may 

 be gathered from the following anecdote told by Palmer 

 of the frigate-bird, found there in large numbers. He says : 

 " Scarcely had I pushed one ofl", when another frigate-bird 

 would rush up, seize the youni; one, fly ofl", and eat it. 

 Sometimes the parent bird would give chase, but it always 

 ended in one or the other eating the young bird. I could 

 scarcely believe my own eyes, so I tried several ; but they 

 would even take young birds out of the nest which were 

 almost fully feathered." 



THE CAUSES OF COLOUR. 



By J. J. Stewart, B.A.Cantab., B.Sc.Lond. 



THE beauty of the objects of nature around us 

 depends so much on the varieties of colour which 

 they exhibit that it becomes an interesting 

 queition, what is the cause of those diilerences '? 

 We have only to think of the blue sky, the green 

 foliage, and the various splendours of flowers and fruits to 

 realize to how great a degree the pleasure we experience 

 in viewing a country scene is due to the many-hued 

 surfaces of natural objects. The pleasurable feelings 

 aroused by the sight of the deep blue of the sea contrast- 

 ing with the bright red colour of the sandstone cliffs, or 

 the eUect of the sunshine when it lights up the splendid 

 purple of the heather on the slopes of the hills, can be 

 forgotten by none who have experienced them ; and the 



striking effect of Eastern scenes is owing largely to 

 the brilliance of the tints which meet the eye. It is 

 needless to enlarge on the part played by colour in this 

 world of ours ; the difference between a world of colour 

 and one in wiiich surrounding objects were only distin- 

 guished by different shades of grey can be realized by all. 



When we come to enquire how the red colour of a rose 

 is produced, and why it differs in appearance from a blue 

 flower, we must consider what happens to the light which 

 falls upon the petals of the rose. We see the flower because 

 the light from the sun is reflected from it, but something 

 has happened to the light before it reaches our eyes ; the 

 light we receive differs from that which fell upon the 

 flower. The rays from the sun penetrate to a certain 

 extent into the substance of the flower, and most of them 

 are reflected from particles beneath the surface. Now the 

 cells making up the petals of a rose contain a fluid which 

 has the power of absorbing certain of the rays of Ught, 

 and the light entering the eye after penetrating a short 

 distance below the surface of the petals and coming back 

 has passed through this fluid, and in its course some of 

 the rays of the sunlight have been abstracted from it. 

 Thus the light reaching us is lacking in certain of the 

 constituents of white light — that is, it is coloured. White 

 light may be considered as made up of the three funda- 

 mental colours, red, green, and violet, blended together. 

 The light which has passed through a certain extent of 

 the substance of the rose petals has been deprived of its 

 green and violet portions, and thus the red rays alone 

 reach our eyes. All substances which possess colour 

 exercise this power of sifting the rays of light. Light 

 falling on the leaves of the rose-bush passes through their 

 superficial layers and is reflected from below the surface ; 

 thus it has to traverse certain particles which take 

 from it the red and violet rays and allow the green to pass. 

 The green rays which escape absorption are the only ones 

 which reach our eyes, and we therefore call the leaves 

 green. 



When white light passes through a prism it is found 

 that in the bind of colour or spectrum produced, the blue 

 and violet rays are the most bent out of their original 

 course ; they are the most refrangible. The red rays are 

 the least bent. It is found also that the vibrations of the 

 ether filling space which produce waves of light are 

 quickest in violet light and slowest in red light. Some 

 substances absorb the quick vibrations more readily and 

 thus appear reldish in colour. Others absorb the slower 

 vibrations, allowing the others to pass through, and there- 

 fore have a green or blue colour. 



When a substance is heated its particles are thrown 

 into a state of rapid motion, and soon set up a motion in 

 the ether which, when the vibrations arc of a certain 

 rapidity, produces light. If a ball of iron is heated it first 

 of all gives out radiation consisting of dark rays which 

 have the effect of producing heat. As its particles become 

 hotter and hotter, and thus vibrate more rapidly, the 

 radiation begins to affect our eyes and the ball glows with 

 a dull red heat. As vibrations of greater and greater 

 rapidity take place owing to the further heating, the ball 

 appears bright yellow, and finally white, when vibrations of 

 all the ditterent rapidities which affect our eyes are given 

 out. In this case it is the motion of the particles or mole- 

 cules of the heated body which is imparted to the ether, 

 and so produces the eflect we call light. Now those mole- 

 cules which are able to execute certain vibrations and give 

 them to the ether, absorb these same vibrations from the 

 ether — that is, absorb certain rays of light. This eflect is 

 best seen in the case of gases, and the phenomenon is 

 analogous to that which occurs with sound. A tuning- 



