Nov. 26, 1 874 I 



NA TURE 



11 



If, however, our rays are circular arcs of unequal curva- 

 tures, we may have crossing, and may also have magnifi- 

 cation or diminution. It is obvious, from Figs. 6 and 7, 

 that to give a magnified virtual image without crossing, 

 the upper ray must be bent downwards more than the 

 lower one ; and that if the lower ray be bent down more 

 than the upper, the image seen will be diminished. 



These rules must be borne in mind in attempting to 

 explain that very common form of mirage in which dis- 

 tant objects are greatly magnified in their vertical dimen- 

 sions, without any other change. Fig. 4 may help us to 

 understand how this magnification arises. If we suppose 

 an object to travel along between two of the rays which 

 proceed from the eye, it is clear from the diagram that 

 the object will begin to be sensibly magnified as it enters 

 the region of rapid change, and the magnification will 

 increase as the object nears the intersection of these rays, 

 at which point it becomes infinite, which practically 

 means that, if placed at this point, it will give rise to an 

 appearance of the greatest possible confusion. As it 

 travels further away between the same two rays it will 

 begin to be again recognised by a highly magnified and 

 inverted image. One of the commonest, I believe the 

 commonest, form of mirage in Australia is one in which 

 small bushes at a distance are magnified into trees ; and 

 I believe the foregoing to be the correct explanation. 



The magnification over water which gives rise to the 

 architectural columns of the Straits of Messina and of the 

 polar regions is more probably to be explained by the 

 action represented in Fig. 6, the region of most rapid 

 change of density being at a height somewhat greater 

 than that of the top of the object, so that the top is 

 greatly elevated by refraction, while the bottom remains 

 nearly in its true place. 



The quasi reflection illustrated in Fig. 4 mny be pro- 

 duced artificially by carefully depositing a;cohol or 

 methylated spirit, to the depth of about an inch, upon 

 water contained in a glass vessel with plane parallel sides. 

 The spirit, though Ughter, has a higher index of refraction 

 than the water ; and at the place of intermixture of the 

 two liquids we have a gradual but very rapid diminution 

 of index in descending. On bringing the eye close to the 

 vessel, and looking obliquely downwards towards this 

 part of the liquid, very perfect inverted images will be 

 seen. The field of view afforded by this arrangement is, 

 however, extremely limited ; and a much finer ettect is ob- 

 tained by the arrangement now before you, in which three 

 liquids are employed, the middle one having the highest 

 index of refraction, while its specific gravity is interme- 

 diate between those of the other two. The three liquids 

 are — (l) A strong solution of alum at the bottom ; (2) 

 pure water at the top ; (3) Scotch whiskey mixed with 

 enough sugar to make its specific gravity intermediate 

 between those of the other two liquids. It is introduced 

 last by means of a pipette. 



Plate III. represents the appearance which this arrange- 

 ment afforded when set up at a window of my house look- 

 ing towards the mountains. 



Every object in the landscape was tripled, the three 

 images being seen at once ; and the vertical breadth of 

 the strip of landscape thus tripled at one view extended 

 from the top of the hills down to the houses on the Lis- 

 burn road. The figure only shows the more conspicuous 

 objects. When the sun was shining on the front of the 

 row of houses represented, which was nearly half a mile 

 distant, I was able to see distinctly the chimneys and 

 windows, and even to see whether the Winds were up, 

 down, or half-way down. It was easy to fancy that the 

 inverted trees and houses were the reflections of the 

 upper ones in water. But a much more striking effect, 

 as of water, was at the place which is left while in the 

 figure, at the junction of the middle and lower image. 

 This had all the appearance of a cahn bay or lake glisten- 

 ing in the sunshine. There are only two natural objects 



to which this peculiar glistening belongs, with brightness 

 far surpassing that of all the dry and solid parts of a 

 landscape. One of these is water, and the other is the 

 sky. A bit of sky has, in fact, been trapped between two 

 portions of land ; and it is a similar trapping of sky in 

 the midst of dry land that produces the irresistible im- 

 pression of a lake of water in the mind of the traveller in 

 the desert. The middle image is probably formed by 

 rays which have taken a path something like those in 

 Figs. I, 2, and 3. The highest and lowest image are 

 formed by rays which have only been bent one way. 



The arrangement of three liquids just described, which 

 was suggested to me by Prof. Clerk- Maxwell, is extremely 

 effective, but requires much delicacy in its preparation to 

 ensure success. 



Triple images of objects below the level of the vessel 

 may be obiained by employing only the two first-men- 

 tioned hquids — alum water and pure water, or strong 

 brine and pure water. A little gentle stirring is advan- 

 tageous whichever arrangement be employed, a glass rod 

 being inserted vertically, passed a few times slowly round 

 the circumferential portion of the liquids, and then 

 withdrawn. 



With the two-liquid arrangement I have obtained three 

 spectra, the middle one inverted, by employing as object 

 a horizontal slit in the shutter of a dark room ; and very 

 brilliant colour effects were obtained by bringing the eye 

 to the conjugate focus of the slit. A screen held at this 

 conjugate focus, which was at first close behind the vessel 

 of liquid, and slowly receded day by day, received an 

 image of the slit very similar to that which would be 

 formed by a cylindrical lens. 



In order to see the three images (or spectra), it was 

 necessary to hold the eye behind the conjugate focus. 

 When it was held in fiont (that is nearer to the vessel), 

 only two images were sesn, sometimes only one, the 

 middle cr inverted image being always wanting. 



A similar lengthening of fucus day by day wj.s obseri'ed 

 with the three-liquid arrangement, which woalJ duubtljss 

 yield similar colour elf.cts. 



ON THE GEOGRAPHICAL DISTRIBUTION OF 

 THE FALLOW DEER IN PRESENT A.\D IN 

 PAST TIME* 



NATURAL History shares with History the doubtful 

 honour of having not a few chapters which are, to 

 use a well-known expression of TaUe\raad, nothing more 

 than " des fables convenues," or which, in fact, contain 

 generally accepted fabrications. To this shadowy side of 

 science Geology gives the largest contributions, but 

 Zoology, especially as regards the habits, habitats, and 

 geographical distribution of animals, is by no means poor 

 in them. Of the Fallow Deer {Cervits dama) it is gene- 

 rally stated in all zoological text-books, " It is a 

 native of the Mediterranean area, and was introduced 

 thence into Germany, Scandinavia, and England, after 

 the Crusades." And yet the Fallow Deer was, many 

 thousand years ago, not only an inhabitant of Africa 

 and Western Asia, but also as much at home in Southern 

 Russia, and even in Central Europe and Denmark, as in 

 Italy and Southern France. 



My researches into the geographical distribution of the 

 Fallow Deer in former epochs have been caused (like 

 those upon the history of the Domestic Fowl t) by a dis- 

 covery in the ancient history of the city of OlmiJtz. I n the 

 same formation as the skull of the fowl there spoken of was 



* By L. H. Jeitteles. Translated from Der Zoologische Gartgn for 

 August 1874. [I have thought it desirable that this article sh»uld be better 

 itnown, as even in such recent works as Mr. Boyd Dawkins' " Cave 

 Hunting," and the new edition of Bell's " British Quadrupeds," the anrient 

 fable ol the Fallow Deer being iniigejuJits only in Southern Europe is re- 

 peated. -P. L. S.J 



t See Der Zoolo^s<;h€ Garten^ bd. xiv. pp. 55 et seq. 



