CHAMBERS'S INFORMATION FOR THE PEOPLE. 



known : Take an empty basin, and place it on a 

 table ; then lay a shilling at the bottom of the 

 basin in such a position that the eye of the ob- 

 server will just not see it Now fill the basin with 



Fig. 9. 



water, and the shilling, though lying unmoved, will 

 come completely into sight. The explanation of 

 this phenomenon is, that the rays of light produc- 

 ing vision in the eye are bent on emerging from 

 the water, and have all the effect of conveying our 

 sight round a corner. In like manner, the refrac- 

 tive power of water is observable when we thrust 

 a straight stick or instrument into it, on aiming at 

 any object. We see that the stick seems to be 

 bent, and fails in reaching the point which we 

 desired it should. On this account, the aim by a 

 person not directly over a fish must be made at a 

 point apparently below it, otherwise the weapon 

 will miss by flying too high. In the same way, 

 objects at the bottom of a river appear higher up, 

 or nearer the surface than they really are, by one- 

 fourth of their actual, or one-third of their apparent 

 depth. A pool which appears to be four and a 

 half feet deep is in reality six feet deep, and fatal 

 accidents have often happened from the ignorance 

 of bathers on this point. 



If the density of the medium changes gradually 

 instead of suddenly, the rays are gradually bent, or 

 curved. The rays by which we see celestial objects 

 pass through the whole thickness of our atmos- 

 phere, which is of extreme tenuity in its highest 

 parts, and of greatest density at the earth's surface, 

 and consequently these rays are bent, as in the 

 diagram. The direction in which they finally strike 



Fig. 10. 



the eye of an observer at O is the line OR, and 

 therefore a star, S, appears to be higher than it 

 really is. If a star is right overhead, or in the 

 zenith, as at Z, its position is not affected by refrac- 

 tion, as the rays by which it is seen fall at right 

 angles on the surfaces which bound media of dif- 

 ferent density. For stars not in the zenith, the 

 observed position must be corrected by subtracting 

 the refraction, and this correction increases with 



244 



the distance from the zenith, only much more 

 rapidly. It is well known that when the disc of 

 the sun or moon is seen near the horizon, it does 

 not appear perfectly round, but seems longer from 

 east to west than from north to south. This is 

 because the southern limb is more raised by refrac- 

 tion than the northern one, while the eastern and 

 western limbs are equally affected. 



Twilight, or the gradual beginning and ending of 

 daylight, is due partly to the refraction of the sun's 

 rays by our atmosphere, but chiefly to their reflec- 

 tion in all directions by the atmospheric particles. 

 If the atmosphere did not exist, we should have the 

 darkness of midnight until the first streak of the 

 sun's disc appeared above the horizon, and then 

 suddenly this would be succeeded by broad day- 

 light, for we learn in total eclipses of the sun that 

 there is nearly full daylight as long as any portion 

 of the disc is visible. But while the sun is still 

 below the horizon, some of his rays striking the 

 higher parts of the atmosphere are reflected down- 

 wards, and so reach the eye, and the more the 

 nearer he is to the horizon ; so that before sunrise 

 we have all possible gradations of light, from the 

 faintest glimmer to the full day. 



It will now be evident that the directness of our 

 vision is at all times liable to be disturbed by 

 atmospheric conditions. So long as the atmos- 

 phere between us and the object at which we are 

 looking is of the same density, we see as it were in 

 a straight line to the object ; but if by any cause a 

 portion of that atmosphere is rendered more or less 

 dense, the line of vision is not straight, or we see 

 the object not in its true place. If the layers of 

 air should increase in density from the earth's sur- 

 face, then objects would appear lower than they 

 really are. In the sandy plains of Africa, the 

 intense heat of the soil at mid-day rarefies the air 

 over it, so that the least dense layers are the lowest 

 Rays, then, from 

 an elevated ob- 

 ject, as A, are 

 bent as in the 

 figure, and a per- 

 son at E will 

 receive the ray 

 as if it came 

 from B. An in- 

 verted image of 

 the object is thus 

 formed, precisely 

 as if it had been 

 reflected from 

 the surface of a lake, 

 called the mirage. 



In order to represent artificially the effects of 

 the mirage, Dr Wollaston suggested the viewing 

 of an object through a stratum of spirit of wine 

 lying above water in a crystal jar, or a stratum of 

 water lying above one of syrup. These substances, 

 by their gradual incorporation, produce a sub- 

 stance whose refractive power diminishes from the 

 spirit of wine to the water, or from the syrup to the 

 water ; so that, by looking through the mixed or 

 intermediate stratum at a word or object held 

 behind the bottle which contains the fluids, an 

 inverted image will be seen. The same effect, it 

 has been shewn, may be produced by looking 

 along the side of a red-hot poker at a word or 

 object ten or twelve feet distant At a distance 

 less than three-eighths of an inch from the line of 



Fig. n. 

 This optical illusion 



