ON THE THEORY OF OPTICS. 



415 



8un that the term focus, meaning a fireplace, was first applied. But if the 

 rays tending to this focus be intercepted, and made to diverge, the point will 

 then be their virtual focus, since they will never actually arrive at it, being 

 made to diverge as if they proceeded from a new point, which will also be a 

 virtual focus. When the divergence or convergence of rays of light is altered 

 by refraction or reflection at any surface, the foci of the incident and re- 

 fracted or reflected rays are called conjugate to each other: the new focus 

 is also called the image of the former focus. Thus, in the case already 

 mentioned, where the convergence of the rays to one focus is converted 

 into divergence from another, the two virtual foci are conjugate to each 

 other; and the original focus of the lens or mirror is conjugate to the place 

 of the sun, or of the luminous object. If the object had been put in the 

 place of its image, the image would then have occupied that of the object; a pror 

 perty which follows from the direct return of every ray of light through the 

 path by which it has arrived, and which may easily be illustrated by experi- 

 mental confirmation. (Plate XXVII. Fig. 375.) 



Whenever light is reflected by a plane surface, the conjugate foci are at 

 equal distances from it, and in the same perpendicular. Thus, every point 

 of an image in a looking glass is perpendicularly opposite to the correspond- 

 ing point of the object, and is at the same distance behind the looking glass, 

 as the point of the object is before it. (Plate XXVII. Fig. 376.) 



The focus into which parallel rays are collected, or from which the}' are 

 made to divei-ge, is called the principal focus of a surface or substance. The 

 sun is so distant, that the rays, proceeding from any point of his surface, 

 aifect our senses as if they were perfectly parallel, and the principal focal 

 distance of a surface or substance may often be practically determined by 

 measuring the distance of the image of the sun, or of any other remote 

 object, which is formed by it. 



In order that the rays of light, proceeding from or towards any one point, 

 may be made to converge by reflection towards another, the form of the surface 

 must be elliptical, parabolic, or hyperbolic; there are also curves of still more 

 intricate forms, which possess the same property with respect to refrac- 



