272 FERGUSON'S LECTURES. 



LECT half as long as himself, sees his whole image as at 

 ^v-^, E F, behind the glass, exactly equal to his own size. 

 For, a ray A C proceeding from his eye at A, and 

 falling perpendicularly upon the surface of the glass at 

 C, is reflected back to his eye in the same line C A ; 

 and the eye of his image will appear at E, in the same 

 line produced to E, beyond the glass. And a ray B D, 

 flowing from his foot, and falling obliquely on the glass 

 at D, will be reflected as obliquely on the other side of 

 the perpendicular a b D, in the direction DA; and the 

 foot of his image will appear at F, in the direction of 

 the reflected ray A D, produced to F, where it is cut 

 by the right line B G F, drawn parallel to the right 

 line ACE. Just the same as if the glass were taken 

 away, and a real man stood at F, equal in size to the 

 man standing at B: for to his eye at A, the eye of the 

 other man at E would be seen in the direction of the 

 line ACE; and the foot of the man at F would be 

 seen by the eye A, in the direction of the line A D F. 



If the glass be brought nearer the man A B, as sup- 

 pose to c b, he will see his image as at CD G: for 

 the reflected ray C A (being perpendicular to the glass) 

 will shew the eye of the image as at C; and the inci- 

 dent ray B b, being reflected in the line b A, will shew 

 the foot of his image as at G / the angle of reflection 

 a b A being always equal to the angle of incidence 

 B b a: and so of all the intermediate rays from A. to B. 

 Hence, if the the man A B advances towards the glass 

 C -D, his image will approach towards it ; and if he 

 recedes from the glass, his image will also recede from 

 it 78 



Note 70. The Kaleidoscope, which consists of a combination of plane 

 mirrors may justly be considered as the most amusir.g of all optical 

 toys ; and, indeed, its application to the ornamental parts of our ma- 

 nufactures, gives it a claim of a still higher nature. 



One of the earliest forms of the Kaleidoscope, was an instrument 

 proposed by Mr. Bradley in 1717. This merely consisted of two plane 



