172 Experimental Philosophy. [Lecture 12. 



able part of modern furniture, as they exhibit a 

 large company, assembled in a room, in a very 

 small compass. Globes lined with amalgam used 

 to be formerly hung up in the middle of a room, 

 by which the whole company were exhibited at 

 one view, seated at a dinner-table, or dispersed 

 about the room. 



The phenomena of concave mirrors are still 

 different. By them convergent rays are ren- 

 dered still more convergent, and consequently 

 the visual angle is enlarged. Their general effect 

 is therefore to magnify. This will be sufficiently 

 exemplified by PL. XIV. fig. 62. In this, as in 

 the former instance, a face is looking at itself; 

 and I take the extreme of those rays which can 

 be reflected to the eye, one from the forehead 

 and one from the chin. These lines, ac, and 

 mn, are reflected to the eye at o, which con- 

 sequently sees the image in the lines of reflection, 

 and in the angle odq, and therefore evidently 

 magnified beyond the natural size, and at a small 

 distance behind the mirror. 



This effect, however, will only take place 

 when the eye is between the mirror and its prin- 

 cipal focus, that is, the focus or point, where 

 rays falling parallel or perpendicular on the glass, 

 will unite after reflection ; the point where the 

 rays of the sun (which are always considered as 

 parallel) will unite and burn: for a concave 

 mirror acts as a burning-glass. By the great law 

 of reflection, the principal focus of a concave 



