SIMPLE EXPERIMENTS luV OPTICS. 



659 



cards flat on a board, one over the other ; measure off a half-inch from 

 one end of the top postal-card, and with the awl punch a hole through 

 them all just Jialf-way from each side. Trim the holes with a pen- 

 knife, and then take one of the cards and one of the wooden slips and 

 put the card squarely on one of the wooden blocks, and, placing the 

 slip over it, tack them both down to the block. Place one of the 

 blocks near a lighted lamp, as shown in the figure, and another at the 



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'16 



FlO. 1. EXPEKIMEXT PEOVIXG THAT LlGHT .MOVES IN STRAIGHT LlNES. 



opposite side of the table, whei-e the observer can sit to look through 

 the aperture. When the light is seen through both openings, draw the 

 third card into line between the others, when the ray will be seen to 

 pass through all three cards. Next, take a piece of thread and stretch 

 it against the sides of the three cards as they stand, and it will be 

 seen that they are exactly in line, and, as the holes in the cards are at 

 the same distance from their edges, it is proved that the beam of light 

 that passes through all the holes must also be straight. If the posi- 

 tion of the blocks is changed, so that the directions of the holes in the 

 cards are different, the same effect will be observed, so that it is de- 

 monstrated that light moves in exactly straight lines in all direc- 

 tions from the source of illumination. Of course, a pupil can learn 

 from a book that light moves in straight lines, but this will be a mat- 

 ter of hearsay or authority, and not of direct knowledge, while if he 

 makes this experiment he will have begun to prove things for him- 

 self, and the preparation for it, and trial in different ways, will be 

 a good exercise in manipulation. 



Now, if the student wishes to prove the variation in the quantity 

 or intensity of light at varying distances, he can do it in the simple 

 way shown in Fig. 2. A small slit is cut in the card near the lamp, 

 through which the light passes, A sheet of white paper, resting 

 ao-ainst some books at the opposite side of the table, forms a screen, 

 upon which the light falls. A bit of paper, an inch square, is 

 held by the point of the awl, the handle of which is stuck in some 



