REFRACTION 



watches. The merchant, with his lens, examines the separate 

 threads of woolen and silk fabrics to determine the strength 

 and value of the material. The physician, with his invaluable 

 microscope, counts the number of infinitesimal corpuscles in 

 the blood and bases his prescription on that count; he ex- 

 amines the sputum of a patient to determine whether tuber- 

 culosis wastes the system. The bacteriologist with a similar 

 instrument scrutinizes the drinking water and learns whether 

 the dangerous typhoid germs are present. The future of 

 medicine will depend somewhat upon the additional secrets 

 which man is able to force from nature through the use of 

 powerful lenses, because as lenses have in the past been the 

 means of revealing disease germs, so in the future more powerful 

 lenses may serve to bring to light germs yet unknown. 



The window pane. We have seen that light is bent when 

 it passes from one medium to another of different density, and 

 that objects viewed by refracted light do not appear in their 

 proper positions. 



When a ray of light passes through a piece of plane glass, 

 such as a window pane, it is refracted and bent slightly from its 

 course toward the perpendicular. When it emerges from the 

 glass, the light is refracted away from the perpendicular and is 

 again bent slightly. Hence, when we view objects through 

 the window, we see them slightly displaced in position, but 

 otherwise unchanged. The displacement caused by glass as 

 thin as window panes is too slight to be noticed, and we are 

 not conscious that objects are out of position. 



Chandelier crystals and prisms. When a ray of light passes 

 through plane glass, like a window pane, it is shifted slightly. 

 But when a beam of light passes through a triangular glass 

 such as a chandelier crystal, its direction is greatly changed, 

 and an object viewed through a prism is seen quite out of 

 its true position. 



