570 



KNOWLEDGE & SCIENTIFIC NEWS. 



[October, 1906. 



plate is most intense near the edges of this image. This 

 ofTcct is r.ttributcd by Rutherford to the extreme thinness of 

 the radioactive layer talvcn in conjunction with the fact that 

 the alpha particles are, on an average, projected equally in 

 all directions. N'arious corroborative experiments have been 

 made. A brass rod of square cross section is made radio- 

 active anil examined either photographically or by its action 

 upon a screen of willemite. If the screen is placed parallel 

 to one of the faces of the rod the appearance is that of a 

 feebly luminous band of about the same width as the rod, 

 bordered on each side by a more luminous region. The 

 centr.al region is made luminous by only one side of the 

 rod, while the outer regions are affected by two sides. 

 W'hen examined photographically the edges of the central 

 band are very sharply defined without any gradual transi- 

 tion of intensity from one region to the other. Still more 

 complicated appearances are obtained by the use of a rod of 

 hexagonal section. A sudden change of intensity occurs at 

 every jioint where the tangential radiation from another 

 side of the rod adds its effect. 



Formulae for Combinations of Lenses. 



In a paper on lens testing read befure the Optical Con- 

 vention by Mr. T. H. Blakesley the following simple 

 formute for lens combinations are given : — 



1. If light passes through two lenses whose focal lengths 

 are fj, i, respectively, and the distance between the second 

 principal focus of the first lens and the first principal focus 

 of the second lens is E, mraau^'cil positirdy along the axis in 

 the direction in which liiiht is tiavellinn, then the resulting 

 focal length of the combinaticin is fr/K. 



2. The first principal focus of the combination is found by 

 moving from the first principal focus of the first lens a 

 distance fi7K in a direction opposed to the direction of 

 light. 



3. A similar motion fo°/K takes pl.ace in the second 

 principal focus, but in this case the motion, if positive, is 

 down the stream of light. 



4. If an object is in the first principal focus of a lens 

 whose focal length is fi and a second lens whose focal length 

 is {-2 is applied to the issuing light, at any distance from the 

 first lens, the magnification of the resulting image is con- 

 stant and its value is — fo/fj. 



In applving these principles a thin concave lens has a 

 positive focal length, a thin convex lens a negative focal 

 length. The magnification is negative when the image is 

 inverted. The lenses are supposed to be coaxial. 



.Although these formula follow at once from well-known 

 principles, thev are pmhably not as generally known as 

 ihev deserve to be. 



ZOOLOGICAL. 



By R. Lydekker. 

 Winter Whitening of Hair. 



.■\s his observations have recently been called in question, it 

 is interesting to note that Professor E. Metchnikoff, in a 

 communication published in the Comptcs Sendus of the 

 Paris Academy for the present year (Vol. CXLII., pp. 1024, 

 et scq.), reiterates the statement that the winter whitening 

 of hairs (as well as feathers) is due to the action of a 

 particular kind of phagocytes, or chromophagous cells, 

 which remove the pigment from the interior of the hairs or 

 feathers to the skin or to the surface. The theory that 

 gas is the bleaching agent will, in the Professor's opinion, 

 not hold good. Mr. Metchnikoff's observations have been 

 made on the hair of the variable hare, and on the feathers 

 of willow-grouse, ptarmigan, and of a hen, whose plumage 

 began to turn white ; and the chromophagous cells are 

 stated to have been actually seen at work. 



The Russian Tarpan. 



In a communication published in the I'roccidings of the 

 Koyal Society of Edinburgh, Professor Ewart expresses the 

 opinion that the now extinct tarpan, or wild horse of the 

 Russian steppes, was not a distinct species or race, but a 

 hybrid produced from the mingling of the Mongolian wild 



horse with certain other ponies. He also ventures on the 

 conjecture that the " Celtic pony " will probably prove more 

 distinct from the horse than is eilher the zebra or the ass. 



The Eye of the Mole-Rat. 



If additional evidence in favour of the evolution of 

 animals be necessary it would be afforded by the observations 

 of an Hungarian anatomist, on the eye of the great mole-rat 

 {Spalax typhlus) of Eastern Europe and Egypt, a creatuie 

 which has the general habits of a mole, but is a strict 

 vegetarian. .Mthough the eye is completely buried beneath 

 the skin (which probably permits the passage of a certain 

 amount of light), and is thus functionless, yet all the 

 essential structural features of that wonderful organ are 

 present — only tin y never develop beyond the initial stages. 



The Multiplication of Generic Names. 



As an instance of the complexity now being introduced 

 into zoological classification by advanced specialists, it may 

 be noted that the well-known American naturalist, Mr. 

 G. S. Miller, has recently proposed no less than twelve new 

 genera of bats, all based on species which had previously 

 been referred to other genera. The same writer is also of 

 opinion that the Philippine flying-lemur, or colugo, should 

 be separated generically from the ordinary Malay species. 

 For the former, he adopts Gray's name, Cohigo, while for 

 the latter, on account of priority, he would employ the 

 name Cymicephalus (long applied to the African baboons), 

 for the familiar and time-honoured Galeopithecus. This 

 change involves the replacement of the family name 

 Oah'upifheriilfp by Cohigid<e. It will be curious to see how 

 naturalists, other than specialists, follow this l<'ad. 



The Habits of Elephant-Seals. 



In an account of the zoological collections brought home 

 from South Georgia, Dr. Einar Liinnberg gives some inter- 

 esting particulars with regard to the habits of that gigantic 

 animal, the elephant-seal, or sea-elephant. When disturbed 

 by man, these monsters, if much irritated, raise themselves 

 on the pelvic region, with the greater part of the body in an 

 upright position, and at the same time inflate their nostrils, 

 open their mouths, and roar hideously. At such times 

 they are dangerous to approach directly in front, although 

 thev are at the same time so heavy and clumsy that an 

 active man, if standing a little to one side, can escape their 

 attack without much difliculty. Contrary to the general 

 belief, they use only the fore-flippers when moving on land. 

 .Mthough the old males are generally silent, the young 

 bulls keep up a constant howling during the spring, which 

 has been compared to the barking of dogs. 



Origin of Donnesticated Fowls. 



In Spolia Zcylanica for July (of which the Editor has been 

 favoured with a copy) some interesting information is given 

 with regard to hybridisation experiments between the Ceylon 

 jungle-fowl and ordinary fowls. It is well known that such 

 hybrids can be produced, but since in the one case at that 

 time recorded such hybrids were infertile inter se, Darwin 

 was led to exclude the wild Ceylon bird from the ancestry of 

 domesticated fowls. In the new experiments, by mating 

 hybrid cocks with domesticated hens, fertile eggs have been 

 produced, as they also have by crossing a hybrid cock with 

 a f domestic hen. Hybrid cocks crossed with hybrid hens 

 have, however, hitherto failed to produce fertile eggs. 



REVIEWS OF BOOKS. 



ASTRONOMY. 

 Royal Astronomical Society of Canada : Transactions for 

 1905, including selected papers and proceedings (Toronto, 

 ic)o6, 209 pp.). — The contents of this volume, extracted from 

 the year's work of an obviously vigorous and flourishing 

 society, cover a wide range, from .American-Indian stellar 

 legends to the new Dominion Observatory at Ottawa and 

 the latest Pacific cable, and from the astronomy of Tenny- 

 son to the figure of the sun. The disappointing eclipse 

 expedition to Labrador is much in evidence, partly on ac- 

 count of elaborate experimental preparations for coronal 



