Sept. 7, 1883.] 



♦ KNOWLEDGE ♦ 



155 



THE COLOURED CURTAIN IN THE EYE. 



By ^YILLIAM ACKROYD. 



THIS ring-like curtain in the eye, of grey, green, bluish- 

 green, brown, and other colours, is one among the 

 very many remarkable contrivances of the organic world. 

 The eye cannot bear too much light entering into it, and 

 the coloured curtain so regulates its own movements that 

 too much light cannot enter the eye. The dark circular 

 aperture in the centre, known as the pupil, is consequently 

 for ever altering its size ; on a bright sunshiny day, out in 

 the open, it may be only the size of a pin's head, but at 

 night, when there is no light stronger than starlight, it is 

 even bigger than a pea. 



This coloured ring curtain is fixed at its outer edge, 

 and its inner edge expands or contracts so readily and, 

 apparently, so easily, preserving its circular outline all the 

 while, that it is quite provoking to the inventor, who has 

 been trying to invent movable "stops" or "diaphragms" 

 for years, and after all his labour cannot even approach it in 

 perfection, and his despair is complete when he learns that 

 the movements of this eye curtain are automatic and quite 

 independent of the will. 



It is unlike the ordinary window-blind, which is gene- 

 rally of a rectangular shape, and is drawn up or let down 

 according to the amount of light entering the room. The 

 eye-curtain or iris is of ring shape, and possesses a wonderful 

 power of expanding itself so as to diminish the area of the 

 pupil, and of shrinking in, so as to enlarge the area of the 

 pupil. Its movements may be watched in a variety of 

 ways, some of which we shall describe. 



Tlie common way of watching the movements of the iris 

 is to regard it closely in a looking-glass while the amount 

 of light entering the eyes is varied. Place yourself before 

 a looking-glass and with your face to the window. Probably 

 the iris will be expanded, and there will only l:ie a very 

 small opening or pupil in the centre. Now shut one eye 

 suddenly, while narrowly watching the other in the glass 

 all the time. At the moment the light is cut off from one 

 eye, the iris of the other contracts or is drawn up so as to 

 enlarge the pupil. This shows that there is a remarkable 

 interdependence between the curtains of the two eyes, as 

 well as that they are afiected by variations in the quantity 

 of light falling on them. 



Perhaps one of the most interesting ways of watching the 

 movements of these sympathetic eye-curtains is one which 

 may be followed while you are outwalking on the street these 

 dark winter nights. A gas-lamp seen at a distance is com- 

 paratively-speaking a point of light, with bars of light 

 emanating from it in many directions. These bars, which 

 give the peculiar spoked appearance to a star, are probably 

 formed by optical defects of the lens within the eye, or by 

 the tear-lluid on the exterior surface of the eye, or by a 

 combination of all these causes. Be that as it may, the 

 lengths of the .spokes of light are limited by the inner 

 margin of the eyt- curtain ; if the curtain be drawn up, then 

 the spokes are long ; if the curtain be let down, or, in 

 other words, if the pupil be very small and contracted, 

 then one cannot see any spokes at all. Hence, as I look at 

 a distant gas-light, with its radiating golden spokes, I am 

 looking at something which will give me a sure indication 

 of any movements of the eye-curtains. I strike a match 

 and allow its light to fall into the eyes ; the spokes of the 

 distant gas-lamp have retreated into the point of flame as 

 if by magic ; as I take the burning match away from 

 before my eyes, the spokes of the gas-lamp venture forth 

 again. The experiment may bo utilised to sec how much 

 light is required to move the window-curtains of the eyes. 



Suppose you are walking towards a couple of gas-lamps, 

 A and B ; B about fifty yards behind A. Then, if you 

 steadfastly look at B and at the golden spokes apparently 

 issuing from it, you may make these spokes a test of how 

 soon the light of A will move your ires. As you gradually 

 approach A, you come at last to a position where its light 

 is strong enough to make the spokes of B begin to shorten ; 

 a little nearer still and they vanish altogether. I have 

 found that about a third of the light which is competent to 

 contract the pupil very markedly will serve to commence 

 its movement. 



Critical Points in Magnetism. — During the last 

 meeting of the Physical Society Professor Hughes brought 

 forward his magnetic experiments recently described by us, 

 and one of the speakers remarked that soft iron brought 

 to a red-hot state loses its property of being attracted by 

 a magnet. As a matter of fact, Professor Hughes finds 

 that this critical state is reached when iron is heated to 

 a yellow-white, or just between white and red heat. Steel 

 also loses its power of being magnetised when heated, and 

 on being cooled to 100 deg. below zero, it again loses it. 

 Nickel, again, loses its magnetic property at a compara- 

 tively low temperature — namely, that of boOing water, 

 and Dr. Crookes has, we lielieve, devised a nickel cut 

 out for electric lighting based on this curious fact. — 

 Engineering. 



In the six months from October 1st, 1880, to April 1st, 

 1881, 4,123 tires broke on sixty-five railroads in Germany, 

 which had 21,2-17 miles of road— 194 to every 100 miles 

 of road, against 2-3 in 1880. The Railroad Gazette points 

 out that there was one tire broken to every 124,760 miles 

 run by a wheel. Like rails in this country, the breakages 

 increase in cold weather. In successive months the numbers 

 were : — October, 3.52 ; December, 43.3 ; January, 1,562 ; 

 February, 775 ; March, 592. Below the freezing-point 

 1,553 breakages occurred ; above it, 956. 



Dynamite Struck by Lightning. — The following well- 

 authenticated case of the firing of a dynamite magazine is 

 of interest : — About a month ago, says the Engineering and 

 Mining Journal of New York, the company operating the 

 salt mines at Avery's, or Petit Anse, Iberia, Parish, La., 

 received a large quantity of dynamite for blasting pur- 

 poses. The same was stored in a magazine situated about 

 four hundred yards from the works. Large trees were in 

 close proximity. On Saturday, July 21st, a few minutes 

 past twelve o'clock, a terrific storm burst on the island. 

 Torrents of rain fell, and vivid Hashes of lightning rent the 

 skies. The workmen had barely left and reached the boarding- 

 house, situated at some distance from the works, when the 

 lightning struck the magazine, and immediately there fol- 

 lowed an awful explosion. For a few minutes all seemed 

 entirely paralysed. Recovering, some rushed out to ascertain 

 the cause. It was then discovered that the whole building 

 in which was stored the explosive compound had dis- 

 appeared, and in its ])lace was left a yawning chasm 15 or 

 20 ft. deep by about 100 ft. in length. The trees surround- 

 ing were prostrated and shattered. One, 2}, ft. in diameter 

 was carried by the immense power in the blast, 200 yards 

 from where it previously stood. Every building on the 

 island contiguous to the mines was shaken in a terrible 

 manner, causing every glass door and window to part with 

 its glass. The building's containing the mills and machinery 

 were much damaged, preventing access to the shaft for the 

 time being. Eight box-cars, ready to be loaded, were 

 entirely crushed in their upper part, the running gear 

 remainina; intact. 



