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THE POPULAR SCIENCE MONTHLY. 



wax on the table. Set the needle-awl, with the bit of paper, about 

 twelve inches from the lamp, and then darken the room. Upon the 

 screen, which is placed two feet from the lamp, will then be seen 

 the shadow of the square bit of paper. With a lead-pencil trace an 

 oiitline of this shadow on the screen, and then move it a foot farther 

 back ; and note how much the shadow is increased in size. With 

 the pencil trace this shadow on the screen, and then laying the paper 



Fig. 2. Experiment with Shadows. 



on the table and measuring the two shadows, you will see how they 

 compare in size, and get a clew to the principle of inverse squares, as 

 it is called. 



Fig. 3 represents the means used in showing that the angle of the 

 ray as it strikes the mirror is the same as that at which it is reflected. 

 A and B are two of the postal-cards and their blocks used in the first 

 experiment, turned Avith their inside faces toward each other, and 

 separated by three more blocks of the same dimensions as those sup- 



Fig. 3. Angle of Incidence and Reflection. 



porting the cards. The flame is placed even with the hole. On the 

 middle block rests a piece of glass, coated on the bottom side with 

 black varnish. The eye looks through the hole H upon the glass, 

 where it sees a small spot of light that is the reflection of the ray 

 from the lamp through the hole A. The point of the needle is placed 



