24 



KNOWLEDGE 



[February 1, 1893. 



In the Antilles a remarkable burrowing petrel, Imown 



as the diablotin, is also believed to be now extinct. ] 



With the handsomely marked Labrador duck {Caiiip- 

 tolmmus lahradoritts) we bring to a close our list of animals 

 which can be pretty definitely affirmed to be extinct, 

 although there are a few others over which the same fate is 

 impcnJlu^', e\cu if it ha>- not already befallen them. This 

 duck was not unlike the common long-tailed duck (Harehhi 

 illacialis) in general coloration and size, although without 

 the long tail of the latter. In the male, the body and 

 primaries are black, and there is also a ring round the 

 neck and a stripe down the head of the same hue ; while 

 all the remainder is white. During the breeding-season this 

 species inhabited Labrador, but in winter its range 

 extended as far south as New Jersey. According to 

 Mr. F. A. Lucas, to whom we are indebted for much 

 information concerning the extinction of several of the 

 species noticed in this article, tlie Labrador duck was 

 never common, and as no example has been seen since 

 1879 it may fairly be presumed to be extinct. 



In conclusion, we may refer to a very remarkable mammal 

 of which but a single example has hitherto come under 

 civilized human ken. As most of our readers are probably 

 acquainted, at least by name, with the large spotted South 

 American rodent, known as the paca — a distant cousin of 

 the familiar guinea pig — we shall assume that they know 

 what we mean when we talk of a paca- like animal. Now, 

 on a certain occasion somewhat more than twenty years 

 ago, the inhabitants of Montana de Vitoc, in Peru, were 

 surprised to find at daybreak a large rodent with the 

 general appearance and coloration of a paca walking 

 unconcernedly about the courtyard of a house. The 

 creature difl'ered, however, from a paca in having a tail of 

 considerable length, as well as in its smaller ears and cleft 

 upper lip ; whUe dissection revealed other internal points 

 of distinction Now the curious part of the matter is that 

 none of the natives of Peru had ever previously heard of or 

 seen a similar animal (for which, by the way, the name of 

 Dimiiiijia was suggested), and from that day to this notliing 

 more has been heard about the creature. Can it be that 

 the specimen then seen and killed was the last survivor of 

 its race, and that the Dinawi/a, whose existence was thus 

 strangely revealed to us, must also be numbered with the 

 extinct ? 



FLUORESCENCE AND PHOSPHORESCENCE. 



By Vaughan Cornish, M.Sc, F.C.S. 



WHEN we new by reflected light the three 

 following objects, a film or leaf of gold, a 

 solution of sulphate of copper, and the leaf 

 of a plant, we see that their colours are 

 yellow or orange, blue, and green respec- 

 tively. If, however, we view these bodies by transmitted 

 light u'.c, look through them at the light), we shall find 

 that the thin gold film ajjpears blue-green instead of 

 yellow, whilst the solution of coi)per sulphate and the leaf 

 of tlie plant appear of the same colours, viz., blue and 

 green, as when seen by reflected light. It will be under- 

 stood without explanation that the light transmitted by a 

 body consists of all or part of the rays which have escaped 

 reflection. In the case of a thin film of gold the rays of 

 the orange end of the spectrum are reflected, •■ and other 



* The action at the surface of a film of gold is evidently somewhat 

 oomplicatud. Tlius polarized liglit falling obliquely on a gold surface 

 varies in colour after reflection with the inclination of the plane of 

 polarization and the plane of irieiilencc. The eiperimeiit is easily 

 tried. If a sovereign be examined with u Nieol's jirism, turned so as 



rays, the blue and green which have escaped reflection, are 

 transmitted through the film. The colours of the object 

 as seen by reflected and by transmitted light are, approxi- 

 mately, complementary colours. 



How is it that in the case of a liquid, such as a solution 

 of copper sulphate, and again in the case of the leaf of a 

 plant, the colour as seen by reflected and by transr.iitted 

 light is the same ? The following experiment will serve 

 to explain the action on light of the solution of copper 

 sulphate. 



The clear solution is pi iced in a vessel the sides and 

 bottom of which are carefully blackened, and the mouth 

 of the vessel is illuminated by ordinary white light. On 

 looking into the vessel the contents appear, not blue, but 

 black. If a blue tinge be distinguished, it is owing to the 

 sides of the vessel not being perfectly protected with the 

 required coating of dull black. In this case the copper 

 sulphate, though illuminated by white light, does not give 

 its well-known bright blue reflection, but reflects practically 

 no light at all. If, however, a little fiuoly-powdered chalk 

 be introduced into the copper sulphate, the contents of the 

 vessel immediately reflect the familiar bright blue colour. 



What happens, obviously, is this : the light falhng on 

 the particles of white chalk is reflected back to the eye of 

 the observer. In its passage into and out of the copper 

 sulphate solution the light has been deprived by absorption 

 of the orange part of the spectrum, and the blue rays alone 

 emerge. Thus the colour of a solution of copper sulphate 

 is the same whether light falls on it or comes through it, 

 because there is, practically, no true reflection at all, but 

 absorption only. When light falls on a vessel containing 

 the liquid the blue colour is due to light from the back- 

 ground, or from solid particles in the liquid ; the orange 

 part of the reflected light being absorbed by the copper 

 sulphate, and blue only passing. We see, therefore, that 

 the colour of copper sulphate is always due to absorption 

 and not to surface reflection. 



In the case of a green leaf which shows the same colour 

 by reflected and by transmitted light, the explanation of 

 what goes on appears to be very similar to the last case. 

 The light penetrates some little way below the surface oi' 

 the leaf, and is there reflected back through the semi- 

 transparent material. The chlorophyll absorbs the red 

 rays and allows only the green to pass out again. With 

 the leaf, as with the copper sulphate, there is no true sur- 

 face reflection, such as is given by the gold film. The 

 colours of most bodies, except the metals, are due to absorp- 

 tion, certain rays being absorbed and extinguished by the 

 body. 



A very interesting class of optical phenomena is pre- 

 sented by certain substances in which the absorbed rays, 

 or some of them, are not extinguished, but are modified so 

 as to emerge with a colour dift'erent from that which they 

 originally possessed. The colour of a ray depends upon 

 the period of vibration, the violet rays vibrating more 

 rapidly than the blue, and the blue more rapidly than the 

 red, which have the longest period of vibration. The rays 

 of colour of shortest vibration period are termed the more 

 refrangible rays from the fact that, in passing through a 

 prism or lens, they are bent further out of their original 

 path than the rays of longer period (the less refrangible), 

 such as the red rays. Substances of the class to which we 

 have referred (which are termed jIuoiviicchI substances) 

 change rays of higher to rays of lower refrangibility, e.y., 

 violet to blue. A good example of the action of such sub- 

 to cut out the light polarized in the plane of incidence, it appears 

 whitish, like a bad shilling; made of lead or pewter; but if the 

 Nicol's prism be turuod at riglit angles to its first position, tlie sove- 

 reign resumes its golden yellow appearance. — A. C. Ranyakd. 



