THE ACTION OF LIGHT 



we must always remember, when dealing with the 

 sun, though, of course, it does not apply when we 

 are dealing with lights near at hand, unless they 

 are specially constructed to throw parallel beams 

 or rays, whichever we elect to call them. To prove 

 that light travels in straight lines is not difficult, 

 and we may devise a number of experiments for the 

 purpose. The doors and ventilators of many dark 

 rooms, in which photographic operations are carried 

 on, are constructed on the assumption that light 

 cannot travel round corners. An arrangement as 

 shown in the diagram will allow air, but no light. 



to pass. If light were capable of going round 

 corners, some other arrangement would have to be 

 devised for the ventilation of dark rooms. 



Having learned so much about light, we come to 

 the most important fact of all, as far as the action 

 of light concerns microscopic work. When rays of 

 light travel, from a substance like air into a sub- 

 stance like water, they are bent out of their straight 

 course. Without any desire to introduce a number 

 of unfamiliar words, we may venture to remark 

 that, any substance through which light passes is 

 called a medium. Some media are clearly more 

 dense, more compact or solid — dense is the proper 



41 



