312 



SCIENTIFIC AMUSEMENTS. 



addition a succession of fainter images becomes visible, these being 

 formed by a repetition of reflections from the two surfaces, just as 

 in the endless gallery. If the mirror be so arranged that the two 

 first reflections give images of about equal brightness, the next 

 reflections will form pairs of images of successively decreasing 

 brightness, because at each reflection (especially at the unsilvered 

 surface) only a portion of the light is actually reflected, some being 

 lost by absorption and some refracted outwards. 



Expt. 346. How to see through a Brick Wall or a Board. 

 Procure 4 equal-sized pieces of flat silvered looking-glass, and 

 arrange them as shown in fig. 154, at angles of 45, with the 

 corners of a square pasteboard or wooden tube shaped in 4 right 

 angles, as shown. The front and back ends of the tube should be 

 closed in, and a hole the size of a shilling cut in the closing in at 

 a and &; opposite to them and behind the two upper mirrors 



should be cut two similar 

 holes, c and d, so that an 

 observer looking in at a 

 apparently sees clear through 

 all 4 holes in a line, so as to 

 view objects beyond at e. 

 Now place a brick, a hat, 

 a piece of board or some 

 similar opaque object, /, 

 between c and d; the ob- 



Fig. 154. To see through a Board. 



server will still see whatever was in view just as well as before, 

 thus apparently seeing straight through the opaque object. 

 The image actually seen is really formed by a 4-fold reflection of 

 such a nature that the rays ultimately reaching the eye practically 

 come in the same direction as those originally emitted by the 

 object viewed, the course of a given ray being along the line 

 indicated. 



Expt. 347. Alteration of Apparent Position by looking 

 through a Medium different from air, with Parallel Faces. 

 It results from the circumstance that the path of a given ray is 

 altered in one direction on passing from air into another medium 

 and in the opposite direction in passing out again (Expt. 333), that 

 on looking through a transparent medium with parallel faces (such 

 as a pane of glass), the rays that meet the eye are usually parallel 

 (or nearly so) with those that would have met the eye had nothing 

 but air intervened, but actually produce the effect of causing the 

 impression that the object seen is slightly displaced. If the light 

 fall absolutely perpendicularly on the glass it will go onwards in 

 exactly the same line through the glass, and subsequently on emerg- 



