22 



CONSTRUCTION OF THE MICROSCOPE. 



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 impinges, requires the intervention of the crystal- 

 line 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, in- 

 deed, the angle it subtends 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. 

 17. 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 there- 

 fore appear nearly twice as long as o w, 

 being seen under twice the angle; and 

 in the same proportion for any greater 

 or lesser difference 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 is- 

 suing from it ; so that they enter the eye in a state of moderate diver- 

 gence, as if they had is- 

 sued from an object be- 

 yond the nearest limit of 

 distinct vision ; and a 

 well-defined image is con- 

 sequently formed upon 

 the retina. In fig. 18 

 a double- convex lens is 

 placed before the eye, 

 near which is a small 

 arrow, to represent the 

 object under examina- 

 tion ; and the cones 



drawn from it are portions of the rays of light diverging from those 

 points and falling upon the lens. These rays, if permitted to fall 



