20 CONSTRUCTION OP THE MICROSCOPE. 



forming what is termed a meniscus, its effect will depend 

 upon the proportion between the two curvatures. 



The rules by which the foci of all lenses may be found, 

 will be more advantageously studied in works on Optics. 



As each ray carries with it the image of the object from 

 whence it proceeded, it follows, that if those rays, after 

 intersecting each other, and having formed an image at 

 their intersection, are again united by refraction or re- 

 flection, they will form a new image, and that repeatedly, 

 so long as their order is not disturbed. It follows, also, 

 that when the course of the luminous ray through several 

 lenses is under consideration, we may look on the image 

 first produced as an object in reference to the second lens, 

 and may consider the second image as produced by this 

 object, and so on successively. This is, indeed, a principle 

 involved in the adaptation of lenses to magnifying objects ; 

 and in fig. 6, it is seen that if the point of light be situated 



Fig. 6. 



above the line of the axis, the focus will then be below it, 

 and vice versd; but the surface of every luminous body may 

 be regarded as comprehending an infinite number of such 

 points, from all of which a pencil of light-rays proceeds, 

 and is refracted according to the general law ; so that 

 a perfect but inverted image or picture of the object is 

 formed upon any surface placed in the focus, and adapted 

 to receive the rays. If any object be placed at twice the 



