346 



ADVANCED PHYSIOLOGY 



Glass 



The law which makes the rays bend as they go through 

 the lens, and causes them to form an image on the other side 

 is very simple. When a ray of light passes from any point 

 through air, it always passes in a straight line; but if it enters 

 a transparent substance like glass, which is denser than air, 

 it is usually turned to one side. If it enters the glass per- 

 pendicularly, it still goes on in a 

 straight line (Fig. 168 C-c); but 

 if it enters at an angle it is always 

 bent to one side, and the greater 

 the angle at which it enters the 

 more it is bent (Fig. 168 A-a, 

 B-b) . After it has passed through 

 the glass and as it comes out on 

 the other side, it is bent again. 



So it happens that rays of light 

 in passing through glass at an 

 acute angle to its surface are sure 

 to change their direction. One 

 often sees this principle illustrated when he looks out of 

 doors through a window. If the light from the objects thus 

 seen comes to him through a more or less irregular piece of 

 ordinary window glass, and especially if it comes through 

 slantwise, the object seems to be very much distorted. If 

 one sees things through a piece of plate glass there appears 

 to be far less disturbance of the image than in the case of un- 

 even glass, yet when the plate glass is bevelled along the edge, 

 objects which can be seen through the general surface plainly, 

 cannot be seen at all, or only in a distorted manner through 

 the bevelled area. Light from the object in going through the 

 bevelled surface has been thrown out of its path. 



Let us now notice the effect of a lens upon rays of light.' 

 The surfaces of the lens are curved, i. e. parts of spheres. 

 From Figure 169 it will be seen that if parallel rays enter the 

 1 For demonstration see Appendix, Sections 32 and 33, 



Air 



FIG. 168. DIAGRAM 



Showing the refraction of light 

 while passing through a piece of 

 glass with parallel surfaces. 



