CONSTRUCTION OP THE MICROSCOPE. 



29 



(s FI) into the new direction (P/). In like manner, the 

 corresponding ray (s F') will be refracted to^ and a white 

 and colourless image there formed by the two lenses. 



The Magnifying Power of Lenses. To assist us in gain- 

 ing a clearer notion of the mode in which a single lens 

 serves to magnify minute objects, it is necessary to take a 

 passing glance at the ordinary phenomena of vision. The 

 human eye is so constituted, that it can only have distinct- 

 vision when the rays falling upon it are parallel or slightly 

 divergent ; because the retina, on which the image im- 

 pinges, requires the intervention of the crystalline lens to 

 bring the rays to an accurate focus upon its surface. The 

 limit of distinct vision is generally estimated at from six 

 to ten inches; objects viewed nearer, to most persons, 

 become indistinct, although they may be larger. The 

 apparent size of an object 

 is, indeed, the angle it sub- 



tends to the 



eye, 



or the 



angle formed by two lines 

 drawn from the centre of 

 the eye to the extremity of 

 the object. This will be 

 understood upon reference 

 to fig. 18. The lines drawn 

 from the eye to A and R form 

 an angle, which, when the 

 distance is small, is nearly 

 twice as great as the angle 

 from the eye to o w, formed by lines drawn at twice the 

 distance. The arrow at A R will therefore appear nearly 

 twice as long as o w, being seen under twice the angle; 

 and in the same proportion for .any greater or less differ- 

 ence in distance. This, then, is called the angle of vision^ 

 or the visual angle. Now the utility of a convex lens 

 interposed between a near object and the eye consists in 

 its reducing the divergence of the rays forming the several 

 pencils issuing from it ; so that they enter the eye in a 

 state of moderate divergence, as if they had issued from 

 an object beyond the nearest limit of distinct vision ; and 

 a well-defined image is consequently formed upon the 

 retina. In fig. 19, a double-convex lens is placed before 



