250 FERGUSON'S LECTURES. 



LECT When parallel rays, as abcdefgh, pass directly 

 VII. 



A 



through a glass A B, which is equally concave on both 

 sides, they will diverge after passing through the glass, 

 as if they had come from a radiant point C, in the cen- 

 ter of the glass's concavity ; which point is called the 

 negative or virtual focus of the glass. Thus the ray a, 

 after passing through the glass A B, will go on in the 

 direction k I, as if it had proceeded from the point C, 

 and no glass been in the way. The ray b will go on in 



Dr. Wollaston has proposed an improvement in microscopes, which he 

 thinks highly advantageous. The great desideratum, he observes, iu 

 employing high magnifiers, is sufficiency of light; and it is accordingly 

 expedient to make the aperture of the little lens as large as is consistent 

 with distinct vision. But if the object to be viewed is of such magnitude 

 as to appear under an angle of several degrees on each side of the center, 

 the requisite distinctness cannot be given to the whole surface by a com- 

 mon lens, in consequence of the confusion occasioned by the oblique 

 incidence of the lateral rays, excepting by means of a very small aper- 

 ture, and proportionable diminution of light. In order to remedy this 

 inconvenience, he used two plano-convex lenses, ground to the same 

 radius, and applied their plane surfaces on opposite sides of the same 

 aperture in a thin piece of metal. Thus he virtually obtained a double 

 convex lens, with this advantage, that the passage of oblique rays was at 

 right angles with the surfaces as well as the central pencil. With a lens 

 BO constructed, the perforation that appeared to give the most perfect dis- 

 tinctness, was about one-fifth part of the focal length in diameter, and 

 when such an aperture is well centred, the visible field is at least as much 

 as twenty degrees in diameter. It is true, that a portion of light is lost 

 oy doubling the number of surfaces, but this is more than compensated 

 by the greater aperture which, under these circumstances, is compatible 

 with distinct vision. 



