270 



FERGUSON'S LECTURES. 



LECT. 

 VII. 



The opera 

 glass. 



object : and so of all the pencils that flow from the 

 intermediate points of the object to the mirror, through 

 the convex glass. 



If a convex glass, of a short focal distance, be placed 

 near the plane mirror, in the end of a short tube, and a 

 convex glass be placed in a hole in the side of the tube, 

 so that the image may be formed between the last-men- 

 tioned convex glass, and the plane mirror ; the image 

 being viewed through this glass will appear magnified. 

 In this manner the opera-glasses are constructed ; 

 with which a gentleman may look at any lady at a dis- 

 tance in the company, and the lady know nothing of it. 



The image of any object that is placed before a plane 

 mirror, appears as big to the eye as the object itself; 

 and is erect, distinct, and seemingly as far behind the 

 mirror, as the object is before it : and that part of the 

 mirror, which reflects the image of the object to the eye 

 (the eye being supposed equally distant from the glass 

 with the object) is just half as long and half as broad 

 as the objeet itself. Let A B be an object placed be- 



fore the reflecting surface g h i of the plane mirror C D; 

 and let the eye be at o. Let A h be a ray of light flow- 

 ing from the top A of the object, and falling upon the 

 mirror at h : and h m be a perpendicular to the surface 

 of the mirror at h : the ray A h will be reflected from 

 the mirror at the eye at o, making an angle mho equal 

 to the angle A hm : then will the top of the image E ap- 

 pear to the eye in the direction of the reflected ray 

 o h produced to E, where the right line A p E, from the 

 top of the object, cuts the right line o h E, at E. Let 

 B i be a ray of light proceeding from the foot of the ob- 



