318 



WELLS'S NATURAL PHILOSOPHY. 



exert different attractive influences on the particles of 

 light coming in contact with them. 



That department of the science of optics 

 which treats of the refraction of li^ht is termed 



What is Diop- 

 trics? 



Dioptrics. 



676. When a ray of light passes through a 

 transparent medium whose sides where tho 

 ray enters and emerges are parallel, it will 

 suffer no permanent change of direction by 



refraction, since the second surface exactly compensates 



for the refractive effect of the first. 



What ensues 

 •when light 

 passes through 

 media with par- 

 allel surfaces ? 



Fig, 256. 



Thus let A A, Fig. 256, be a plate of 

 glass, -n-hose sides are parallel, and B C a 

 ray of light incident upon it ; it will be re- 

 fracted in the direction C D, and on leaving 

 the glass will be refracted again, emerging 

 in the line D E, parallel to the course it 

 •would have pursued if it had not been re- 

 fracted at all, and which is shown by the 

 dotted line. A small lateral displacement is, 

 however, occasioned in the path of the ray, 

 depending on the tliickness of the glass 

 plate. 



This explains the reason why a plato of 

 glass in a window whose surfaces are perfectly parallel, occasions no distor- 

 tion, or alteration of the position of objects seen through it, by reason of its 

 refractive power. The rays suffer two refractions in contrary directions, which 

 produce the same effect as if no refraction had taken place. 



If the surfaces of the medium through which 

 light passes are not parallel, the direction of 

 every ray passing through it is permanently 

 altered, the change being greater as the inch- 

 nation of the two surfaces is greater. 



Thus window-glass of unequal thickness displaces and distorts all objects 

 seen through it. Hence the singular distortion of objects viewed through that 

 swelling, or lump of glass known as the " bull's eye," which is sometimes 

 seen in the center of very coarse panes of glass, and which remains where 

 the glass-blower's instrument was attached. 



677. Any glass having two plane surfaces 

 not parallel^ is called a Prism. 



What happens 

 when light 

 passes through 

 media whos« 

 surfaces are not 

 parallel ? 



What is 

 Prism? 



