LECTURE XIII. 



EXPERIMENTAL PHILOSOPHY. 



VISION AND OPTICAL GLASSES. 



IT has already been explained, that objects 

 are rendered visible not by single rays, but by 

 small bundles of rays diverging from every point 

 of the object, like an inverted cone, or like a 

 painter's brush or pencil, and therefore called 

 pencils of light. It has also been intimated, that 

 these pencils of light are, by the refractive powers 

 of the eye, again made to converge upon the back 

 part of that organ, in points corresponding to 

 those from which they proceeded, so as to form 

 there a complete image of the object. In the 

 tenth lecture, fig. 46, it was further shown, that 

 pencils of light are sent forth in all directions, 

 from every part of a visible object; so that an 

 eye, when placed in any situation that light can 

 travel to it from the object in a straight line, 

 (whether above or below, or at either side) shall 

 be able to perceive it. 



In describing the nature of refraction, enough 

 has been said to show you that it is the property 

 of every convex glass to cause the rays of light 

 to converge. In this respect the eye is to be 



