30 DISCOURSE ON THE STUDY 



give an example : every body knows that objects 

 viewed through a transparent medium, such as water 

 or glass, appear distorted or displaced. Thus, a stick 

 in water appears bent, and an object seen through a 

 prism or wedge of glass seems to be thrown aside 

 from its true place. This effect is owing to what is 

 called the refraction of light ; and a simple rule dis- 

 covered by Willebrod Snell enables any one to say 

 exactly how much the stick will be bent, and how 

 far, and in what direction, the apparent situation of 

 an object seen through the glass will deviate from the 

 real one. If a shilling be laid at the bottom of a 

 basin of water and viewed obliquely, it will appear 

 to be raised by the water ; if instead of water spirits 

 of wine be used it will appear more raised ; if oil, still 

 more: but in none of these cases will it appear to 

 be thrown aside to the right or left of its true place, 

 however the eye be situated. The p/ane, in which 

 are contained the eye, the object, and the point in 

 the surface of the liquid at which the object is seen, 

 is an upright or vertical plane ; and this is one of the 

 principal characters in the ordinary refraction of light, 

 viz. that the ray by which we see an object through a 

 refracting surface, although it undergoes a bending, 

 and is, as it were, broken at the surface, yet, in pur- 

 suing its course to the eye, does not quit a plane 

 perpendicular to the refracting surface. But there 

 are again other substances, such as rock-crystal, and 

 especially Iceland spar, which possess the singular 

 property of doubling the image or appearance of an 

 object seen through them in certain directions ; so 

 that instead of seeing one object we see two, side by 

 side, when such a crystal or spar is interposed be- 



