296 SCIENTIFIC AMUSEMENTS. 



the outside (above the water) appears raised in position : e.g., the 

 experiment with a coin in a cup of water just described. On 

 looking somewhat obliquely into a pool of clean water, the stones, 

 &c., at the bottom will be similarly affected, so that the water appears 

 considerably less deep than it really is ; a matter that should be 

 borne in mind by persons who cannot swim before plunging into 

 water, as it will generally happen that the water is deep enough 

 to drown, if, when looked at from the bank, it appears to be 

 only breast deep or shoulder deep when no allowance is made for 

 refraction. 



Expt. 334. Another Illustration. A long straight stick or 

 pole, thrust obliquely into clear water and viewed from above, will 

 appear to be bent at the surface of the water to an angle, on 

 account of the apparent raising of the submerged part. When, 

 by reason of refraction, reflection, or otherwise, an object is 

 apparently seen to be in a position different from its true one, in 

 virtue of the rays that actually meet the eye passing thereto in 

 directions different from those in which they were actually emitted, 

 what is seen is termed an image.* Thus the appearance of the 

 coin in the cup of water (fig. 136) is due to the formation of an 

 image of the coin at n ; similarly the apparently raised bottom of 

 a pond viewed from above, and the seemingly bent stick partly 

 immersed in water, are due to the formation of images of the 

 stones, &c., and the stick. 



Expt. 335. To illustrate the Absorption of Light. Whenever 

 light is reflected at the surface of, or passes through, a medium, 

 more or less is always absorbed during reflection or passage ; those 

 media which absorb all kinds of light so greatly that practically 

 none at all passes through are said to be opaque. Unless the 

 thickness of the layer of such a medium traversed by the light is 

 extremely small, so little passes through as to be invisible to the 

 eye ; thus a plate of gold or silver is absolutely opaque, even when 

 no thicker than ordinary writing paper; but if beaten out ex- 

 tremely thin into gold leaf or silver leaf, it becomes translucent, 

 allowing some light to pass. In the case of gold the light which 

 passes is of a greenish tint, showing that the absorption of rays 

 other than those producing the sensation of greenness is greatest. 

 In the same kind of w^ay, a coloured transparent medium (such as 

 clear claret or port wine, aqueous sulphate of copper or indigo 



* Certain kinds of lenses and curved mirrors (Expts. 363 and 366) are capable 

 of forming images that can be received on a screen like the glass plate of a 

 camera, or the strained sheet used for a magic lantern ; these are spoken of 

 as real images ; images that cannot be so received on a screen (like the 

 reflection in a plane mirror) are called virtual images. 



