PRISM. 



313 



Fig. 155. Effect of transparent plate. 



ing again into the air ; but if it fall at all obliquely, the path on 

 emerging will only be parallel, and not identical with the original 

 direction. 



Arrange two pieces of straight metal tubing about 3 inches long 

 and ^ inch bore (or two pieces of quill glass tubing with brown 

 paper pasted outside), holding them by stands and clamps in such 

 a way that whilst the axes of the tubes are parallel to one another 

 and in the same vertical plane they are not in the same straight 

 line (fig. 155). Behind one of the tubes place a sheet of card- 

 board, ab, with a hole 

 perforated in it just op- 

 posite the end of the tube 

 at a, and behind this hole 

 place a candle, c. If now 

 you attempt to look 

 through the other tube, 

 placing the eye at d, you 

 will not be able to see 

 the candle ; any rays of 

 light passing through the 

 hole in the pasteboard screen and then traversing the first tube 

 will obviously be unable to pass through the second tube so as to 

 reach the eye. Now place between the two tubes a thick piece 

 of glass with parallel sides (a piece of plate glass, &c.) in the posi- 

 tion indicated, ef. If the thickness and inclination of the glass 

 plate be properly adjusted, as also the relative positions of the two 

 tubes, it will now be possible to see through the whole combination, 

 a ray of light from beyond the screen passing through the orifice 

 in the screen and the front tube, and being then made to deviate 

 to a parallel path by the refractive action of the glass plate, so as 

 to pass through the second tube along the line aghd. 



Expt. 348. Alteration of Apparent Position by looking 

 through a Medium different from Air, with Faces not 

 Parallel. If, instead of a glass plate, 

 a beam of light be made to traverse a 

 prism of glass (i.e., a mass of glass 

 bounded by plane surfaces not paral- 

 lel), the direction in which the light 

 emerges will not only be displaced as 

 compared with the original path, but, 

 further, it will be no longer parallel 

 with the former direction. Fig. 156 

 represents the path of a ray of light, ab, passing through a 

 prism; at the first surface the light is refracted along be, towards 



Fig. 156. Refraction through 

 Prism. 



