Si 



Microscopical Essays. 



The point of union of the rays of light, formed by means of 

 a glafs lens, &c. is called the focus. 



• Now, as each ray carries with it the image of the object, from 

 whence it proceeded, it follows, that if thofe rays, after inter- 

 feeling each other, and having formed an image at their inter- 

 feron, are again united by a refraction or reflection, they will 

 form a new image, and that repeatedly, as long as their order is. 

 not confounded or difturbed. 



It follows alfo, that when the progrefs of the luminous ray is 

 under confideration, we may look on the image as the object, 

 and the object as the image ; and confider the fecond image, a& 

 if it had been produced by the firffc as an object, and fo on. 



In order to gain a clear idea of the wonderful effects produced 

 by glaffes, we muff proceed to fay fomething of the principles of 

 refraction. 



Any body, which is fo conftituted as to yield a paffage to the 

 rays of light, is called a medium. Air, water, glafs, Sec. are 

 mediums of light. If any medium affords an eafy paffage to the 

 rays of light, it is called a rare medium; but if it does not 

 afford an eafy paffage to thefe rays, it is called a dense 

 medium. 



Let Z, Fig. 2, Plate I. be a rare medium, and Y a denfe one ; 

 and let them be feparated by the plane furface G H. Let I K 

 be a perpendicular to it, and cutting it in C. 



