SENSE OF SIGHT. 

 Fig. 7. 



203 



The vitreous humour and crystalline lens magnified, with the stains 

 of the pigmentum nigrum left by the ciliary processes. 



22. Laws of Vision. 4Light passes through the air, or 

 any medium of the same density, in straight lines ; but 

 when it passes from one medium into another, it is refracted, 

 or bent out of a straight Course, unless it strikes the new 

 medium in a perpendicular direction, when it passes directly 

 through. Air, water, glass, or any substance through which 

 light passes, is called a medium. When a ray of light passes 

 from a thinner or rarer medium, into one more dense, as 

 from air into water, it is bent towards a line drawn perpen- 

 dicularly to its surface. The contrary is the case when the 

 reverse happens. This is shown by plunging a straight 

 stick into the water, which will appear crooked. This may 

 be illustrated by the familiar experiment of taking an object, 

 such as a shilling, and fixing it at the bottom of an empty 

 basin, then retiring backwards until the brim of the basin 

 hides it ; then let water be poured into the vessel, and the 

 coin will again come to light, as in the following cut, (See 

 Fig. 8.) 



Thus the ray of light from the coin will proceed in the di- 

 rection #, previous to the addition of the water, but when 

 water is poured into the vessel, the ray will be reflected 

 down to the eye, and the shilling will appear to occupy the 



