No. 1072, Vol. 42] 



NATURE 



flame within a globular opal shade, placed at such a distance that 

 the three images of it produced by the action of the prism (the 

 centre image formed, of course, by the superposition of two, 

 similarly polarized) just touch each other. Two of these images 

 are then filled (like a lantern-disk) with the complementary ring- 

 systems ; and by a very slight motion of the crystal the rings pass 

 from a given disk to the adjacent one, becoming complementary 

 in so doing. (It is hardly necessary to explain, for no doubt 

 Prof. Thompson did so fully, that the whole prism is precisely 



Fig. 2. 



equivalent to a train of two double-image prisms with refracting 

 angles of 45°, having between (hem a plate of spar with surfaces 

 at right angles to the optic axis ; a " Huyghens apparatus," in 

 fact, with an interposed spar-plate instead of the usual selenite 

 film.) 



I may add that I have found it convenient, in order to demon- 

 strate the principle of the prism, to divide it into halves ; or, 

 more strictly, to cut a piece of spar so as to form one half of the 

 prism only, as shown in Fig. 3. Then, if common light from 



Fig. 3. 



the lamp-shad e (as described above) is allowed to enter the face 

 A, and a tourmaline is held in the path of the rays emergent 

 from c, ring-systems are seen just as when a double-image prism 

 is used as a polarizer and a plate of spar held in front of it. 

 Also, if plane- polarized light is allowed to enter c, and the eye 

 is held close to A, ring-.^^ystems are seen side by side ; that 

 portion of the spar through which the rays pass after total 

 reflection at B acting, of course, exactly as a double-image 

 analyzer. In fact, the prism may, in this position, be used 

 alone as a " Savart's polariscope " to detect traces of polarization 

 in sky-light, &c. But for this application, the prism would 

 possess, in the eyes of the true votary of science, the inestimable 

 value of being of no practical utility whatever. 



Queen's College, Oxford. H. G. Madan. 



Coral Reefs, Fossil and Recent. 



Dr. von Le.n'denfei-d's account of the dolomites of the 

 Italian Tyrol, in his letter on " Coral Reefs, Fossil and Recent," 

 is a very valuable contribution to this interesting question ; but 

 I think he can hardly have fallen in with the new edition of 

 Darwin's "Coral Reefs," or he would not have asserted that in 

 the discussion " the structure of our Triassic limestone mountains 

 has been left out of account." In the appendix (p. 332) I 

 wrote : — "If those geologists are right who consider the Schlern 

 dolomites as being to a great extent due to reef-building corals, 

 we have, in the Triassic deposits of the Italian Tyrol, reefs 

 thick enough to satisfy the most exacting requirements." I could 

 not venture upon a more positive statement, because I knew 

 controversy on this question was not ended, and I had not 

 myself, though fairly familiar with the "Dolomites," discovered 

 evidence which appeared to me conclusive (though I incline to 

 the above opinion myselQ, and because I considered that the 

 viesv advanced several years since by Richthofen required some 

 modification— indeed, as to one detail, if I understand him 

 rightly, I should difier from Dr. von Lendenfeld. 



J am confirmed in my idea that he has not read this book, 

 because I find that one of his chief arguments — that against the 

 indefinite lateral extension of a coral reef on a talus of its own 

 building — appears to correspond with one advanced by myself on 

 p. 327, differing only in the addition of an arithmetical example ; 



one of which, indeed, I did work out, but afterwards suppressed 

 as needless, the truth of the statement being obvious when it 

 was once pointed out. T. G. Bonney. 



Bison and Aurochs. 



In regard to Prof. Newton's letter in your issue of the 8th, I 

 beg to state that in restricting the name aurochs to the European 

 bison, I have merely followed the general custom of English 

 zoologists. 



Citing a few authorities, I may first make the following ex- 

 tract from a paper by Prof. W. B. Dawkins, published in the 

 Quart. Journ. Geo!. Soc, vol. xxii. p. 394 (1866). There, 

 after alluding to the Indian gaur, this author writes, "the term 

 Aurochs has been restricted to the European bison by the 

 authority of Buff'on, Cuvier, and Prof Owen ; the term Urox or 

 Bos WHS, to the species under consideration [the extinct wild 

 ox of Europe] by Julius Caesar, Pliny, . . . also by Cuvier, 

 Nilsson, and our great naturalist. Prof. Owen." 



Again, in the article on Ruminants by the late Prof. Garrod 

 in " Cassell's Natural History " (2nd ed., p. 35), the term 

 aurochs is applied to the European bison. Finally, we find in 

 Prof Flower's " Catalogue of Mammalia in the Museum of the 

 College of Surgeons," p. 232 (1884), the animal in question 

 mentioned as the European Bison or Aurochs. 



I find, however, that modern German zoologists (see Brehm's 

 " Thierleben," vol. iii. p. 386) consider it proved that the 

 name Aurochs belongs properly to the extinct Bos primigenius ; 

 and they term the bison, as Prof. Newton states, the Wisunt. 

 If this be really correct, English zoologists must accept the 

 emendation. R. Lydekker. 



The Lodge, Harpenden, Herts. 



The Haunts of the Gorilla. 



Concerning Mr. Du Chaillu's saying (see Nature, May i, 

 p. 19) " that, so far as he is aware, no white man has been able 

 since his time to penetrate to the haunts of the gorilla and bring 

 home specimens killed by himself," I beg to remark that Herr 

 von Koppenfels, in the years 1873-80, killed personally a 

 number of gorillas in the environs of the Ogowe, and sent 3 large 

 specimens, with their skeletons, to the Dresden Museum, some 

 of which I described in the Mittheilungen atis dcm konigl. zoo- 

 logischcn Museum zu Dresden, \o\. ii. 1877, p. 230 .r^^. The 

 Museum in Stuttgart also contains several specimens killed by 

 that intrepid traveller ; and other museums, I believe — American 

 museums, for instance — possess some. (See also his remarks in 

 the American Naturalist, vol. xv. p. 447 ; and Die Gartenlaube, 

 1877, p. 416 scq., with plate ; as well as mine in Der zoologische 

 Garten, 1881, vol. xxii. p. 231.) Herr von Koppenfels, who 

 died in the year 1884 in Erfurt, in consequence of diseases 

 acquired in the tropical climate, says (I.e.) that the haunts of the 

 gorilla in West Africa are in the forests between the mouths of 

 the Mimi and the Congo Rivers, i.e. between 1° N. lat. and 6" 

 S. lat. How far the region extends into the interior is even yet 

 unknown. A. B. Meyer. 



Royal Zoological Museum, Dresden, May 7. 



Flat-fishes. 

 Mr. Gulick, in Nature, vol. xli. p. 537, has raised 

 a puzzling point about the flat-fish. In the case of his two 

 Japanese species, it might appear that the ancestor of them both 

 varied in the two directions as to the position of its eyes, &c., 

 and that by the segregation of each form, neither of which had 

 any advantage over the other, two species eventually were evolved. 

 But this is not so clear in other cases, apparently. On the 

 American coast of the Pacific, there is a flat-fish, Paralichthys 

 californicus, Ayres, which is said by Messrs. Jordan and Goss to 

 be almost as frequently dextral as sinistral. Here, then, is the 

 same sort of variation exactly, yet we see no evidence of segre- 

 gation and the formation of new species. In the whole sub- 

 family Soleince, the eyes and colour are on the right side : now, 

 if the "dextral" soles segregated themselves, having no 

 advantage in being dextral rather than sinistral, what has 

 become of all the sinistral ones ? If there was no natural 

 selection at play, ought we not to get some sinistral species of 

 Solea ? Perhaps it may be said that Solea, as such, never varied 

 in this way, and was always dextral. But this cannot be so, 

 since we have it on Day's authority that the common sole has a 

 reversed aberration. But, after all, the allied Cynoglossina are 

 sinistral soles. 



