DOUBLE IMAGES FORMED BY MIRRORS. 309 



In the ordinary way of using a looking glass the incident 

 rays are not far from perpendicular upon the surface, so that 

 the angle of incidence is very small; consequently the two 

 reflections are nearly superposed ; whilst, moreover, the one 

 from the metal coating at the back is far brighter than the one 

 from the front surface of the glass plate, so that only one image 

 is noticed. 



But it is otherwise when an object is looked at obliquely in 

 such a mirror, more especially if the plate of silvered glass is 

 tolerably thick. Thus, fig. 150 represents a candle flame seen 

 obliquely in such a mirror; one ray, 

 AB, is reflected from the outer sur- 

 face of the glass at B and meets the eye 

 at C, producing the same effect as 

 though the candle had been situated 

 at G ; in other words, forming a vir- G-'' '*' 

 tual image at G. Another ray strikes H ' 



the glass at D and is refracted along ^ 15 n - Double Image of 

 the line DE; at E it is reflected 



from the common surface of the metal film and the back portion 

 of the glass, so as to traverse the line EF; at F it emerges 

 again into the air and is refracted along FC, meeting the eye at C, 

 and producing the same effect as though it had originated from 

 the point H, i.e., forming a second virtual image at H. Hence 

 the eye perceives two reflections of the candle ; the more obliquely 

 the rays fall (i.e., the greater the angle of incidence), the greater 

 is the amount of light passing along the track ABC relatively to 

 the amount passing along the other track, ADEFC. With a 

 small angle of incidence the reflected image, G, is so faint compared 

 with the other one, H, that it is practically imperceptible ; but as 

 the angle of incidence is increased by suitably moving the candle 

 flame, and the position of the eye correspondingly varied so as to 

 see the reflections, it will be noticed that the image G grows 

 brighter and brighter relatively to H which gets fainter ; by and 

 by with considerably oblique rays G becomes the brighter of the 

 two, and ultimately H becomes so faint as to disappear entirely 

 (vide Expt. 345). 



Expt. 344. The Kaleidoscope. In Expt. 341, the effect of 

 two flat vertical mirrors inclined to one another at right angles 

 was obviously to reflect the walls of the cabinet in such a way as 

 to produce virtual images behind the mirrors of objects really 

 situated in front. The kaleidoscope is another application of the 

 same principle, only in this case the mirrors are usually inclined 

 to one another at a smaller angle than 90, and the object to be 



