February i, 19 12] 



NATURE 



447 



(mottled with gold. The surface of the mirror is raised in 

 low ridges, which radiate outwards from the centre in 

 waving lines, like the sun's rays on the Japanese flag. 

 After a few minutes' exposure to air or water a film of 

 metallic green and blue invades the silver. But the 

 .tapetum does not line the whole of the back of the eye. 

 It is bounded by a sharp irregular line which crosses the 

 wall of the globe horizontally, about i mm. below the 

 level of its equator. The lower portion of the retina, a 

 little less than a quarter sphere, is backed, as it is every- 

 where in the human eye, by dense black pigment. Colonel 

 Herschel observes that a man's eye does not gleam. 



In a superbly illustrated monograph published in the 



Philosophical Transactions of the Royal Society Dr. 



Lindsay Johnson pictured the eyes of a great number of 



the animals in the Zoological Gardens as he saw them 



with an ophthalmoscope. With the exception of monkeys, 



the elephant, rhinoceros, and hyrax, all terrestrial 



mammals are provided with a tapetum. The exceptions 



are significant. In monkeys, as in ourselves, the retina 



•shows a "yellow spot." They trust to direct vision with 



its minute discrimination of detail. They move their eyes 



•with great rapidity towards the objects which they wish 



to examine. Such rapid movement is incompatible with 



i«extreme sensitiveness to the movements of external objects. 



*A cat does not move its eyes. It moves its head. Of 



lyrax I have nothing to say, but the elephant and the 



_ rhinoceros stand alone amongst Herbivora. They alone 



'.are indifferent to the movements of lurking enemies — 



great cats and snakes. They do not need to sacrifice visual 



i-precision, as it must be sacrificed in animals in which 



l^he retina is backed by a mirror, in favour of a capacity 



■Df detecting movement. 



I have examined the eyes of a considerable number of 

 »nimals, and find that the disposition of the tapetum, con- 

 sidered in its relation to the habits of the animal, is in 

 %a\\ cases in harmony with the view as to its purpose which 

 ,1 have here expressed. I am also prepared to give an 

 |explanation of the optics of its relation to the retina, but 

 for this or for special illustrations I must not trespass 

 ■upon your space. Alex Hill. 



Colonel Herscmel's letter in Nature of January i8, 

 "followed by that of Mr. Hunt, have no doubt interested 

 others besides myself. I do not think that there is any 

 reason to suppose that any animal's eyes are " auto- 

 phanous," however general the belief to the contrary may 

 be among those not given to accurate observation. I can 

 add to the list of the apparently autophanous the spring- 

 "haas in South Africa and the common English mouse. I 

 generally encourage a few of the latter, and at the present 

 time three have taken up their abode with me in Victoria 

 Street. There is a regulator clock standing if inches 

 away from the wall, and about 6 feet high. I put a little 

 food on the top of the clock, and sometimes behind the 

 clock not quite so high, and in other awkward places. 

 The mice jump on to the skirting board, and there spread 

 themselves out sideways so as to stretch the if inches, and 

 then proceed to go up at an angle of about 40°, climbing, 

 so to speak, a staircase that is not there, and then when 

 this brings them to the side of the clock they turn over 

 in a nimble way with a jump, not always successfully, and 

 negotiate the next flight, and so zigzag to the goal. I 

 often watch these quite close, holding a metallic filament 

 •electric light with shade, so that they are fully illuminated 

 and I am in shade. So long as I am quiet or move slowly, 

 •doing nothing spasmodically, they take no notice. I have 

 evfn prodded and moved the food they were eating with 

 the slide of a long rule, which seemed to perplex rather 

 than frighten them. They do not seem to hear loud noises 

 or singing provided they do not contain S, K, or other 

 sudden sounds, even though I am not a yard away. A 

 few days ago while writing I heard one at work on some 

 bread about 4 feet from the ground, when, to see him 

 "better without getting up, I focusscd the filaments of the 

 electric lamp upon him with a large reading-glass. The 

 mouse did not seem in the least frightened, but stared at 

 the lens a short time, and then I saw his eyes shining with 

 a pale ruby, or rather spinel, colour, and was reminded of 

 Colonel Herschel's letter. 



The same action which makes animals' ej'es appear 

 " autophanous " is seen in far greater perfection in that 

 invaluable little instrument called the reflex light, now used 

 in large numbers to protect the bicyclist from being run 

 down by a motor-car at night. The bicyclist's lamp, of 

 course, is useless, as its light in the road is outshone by 

 that of the motor-lamps, and the lamp itself is generally 

 not directly visible from behind. The bicycle and rider, too, 

 are often by no means conspicuous objects, and the danger 

 of being run down is a real one. To meet this, the reflex 

 light l^as been invented. It may be considered to be a 

 glorified cat's eye. All that is visible from the outside 

 is a ruby bull's-eye lens, but inside, in the principal focus 

 of this lens, there is placed a concave silvered reflector of 

 half the focal length, i.e. the bull's-eye is at its centre of 

 curvature. Any strong light shining upon the bull's eye is 

 therefore brought to a focus on the surface of the mirror, 

 and whether the light is directly in front of the lens, or 

 on one side even to a surprising degree, the focussed light 

 falls normally upon a portion of the reflector, which sends 

 it back to the lens, and so in a parallel beam in the direc- 

 tion from which it came. The driver of the car and his 

 lamps subtend so small an angle at a distance of, say, 

 200 yards, or very much less, that the reflected light is 

 seen by the driver like a red lamp. The committee of 

 the Associated Automobile Clubs were so impressed with 

 the value of this device that the technical committee 

 of the Royal Automobile Club, of which I am a 

 member, examined and tested the reflex light, and they 

 issued a certificate endorsing the claims made for 't. 

 There is a feeling that every cart in the country should 

 carry one, which, unlike ordinary lamps, would entail no 

 trouble or running expense, and would be free from all 

 risk of fire. This reflex light beats any cat's eye or 

 other animal's eye, but it is not autophanous. 



C. V. Boys. 



The experiments described by Colonel Herschel in 

 Nature of January 18 illustrate the reflecting power of 

 animals' eyes, and give no support to the general view 

 that the eyes of cats and dogs " shine in the dark," that 

 is, in the absence of any external source of luminosity. 

 The principle of the experiments is illustrated by the Reflex 

 Lamp commonly fixed at the back of the frame of a bicycle 

 in rural districts. This is not really a lamp, but a bull's- 

 eye of ruby glass about 2 inches in diameter, fixed with 

 the convex surface directed behind the bicycle. When a 

 carriage or motor is approaching the bicycle from behind, 

 its lamps illuminate the bull's-eye, and the reflection is 

 so clear that the driver knows a cyclist is in front of him 

 long before the rider or the machine can be seen. The 

 candle-light used in ordinary carriage lamps enables the 

 Reflex Lamp to be visible at a distance of a hundred yards 

 or so on a dark night. The conditions are precisely 

 similar to those described by Colonel Herschel, the only 

 difference being that a glass convex lens takes the place 

 of the animals' eyes. R. A. G. 



January 27. 



Glazed Frost ; a Reminiscence. 



Mr. Harding's letter (Nature, Januar>' 25, p. 414) 

 reminds me of an experience which, in view of the rarity 

 of the phenomenon, may be of sufficient interest to place 

 upon record in these columns, although the newspapers of 

 the period— the sixties of last century— duly noted the occur- 

 rence. It must, I think, have been in 1866 or 1867 (date and 

 year uncertain) that I had occasion to go from the West to 

 the East End of London. Starting upon my journey about 

 10 p.m., it began to rain soon after I left the house in 

 Bayswater, and I opened an umbrella, which, to my sur- 

 prise, became stiffer and heavier every moment, and was 

 found on examination to be so thickly glazed over with 

 ice that it was impossible to close it. .At the same time 

 the pavements and roadway were also becoming uniformly 

 glazed ; pedestrian movement was most difficult, and all 

 horse traffic was suspended. Although an experience of 

 some forty-five years ago. the impression left upon my 

 memory is still vivid— the ludicrous sight of people carry- 

 ing ponderous and rigidly frozen umbrellas which they 

 could not close, the stream of skaters down Oxford Street 



NO. 2205, VOL. 88] 



