THE MICROSCOPE. 345 



that on issuing from it they converge, and at a certain distance behind 

 it are collected into a small image of the object; the distance being so 

 much less as the curvature of the surface is greater and the form of the 

 lens approximates to that of a sphere. The place at which this image 

 appears is the focus of the lens. If the body from which the rays 

 proceed be placed in front of the lens at the distance of its focus, the 

 inflection of the rays which fall upon the anterior surface in divergent 

 lines, will cause them to issue on the other side in parallel lines. 

 Move the object still nearer, so that the angle of divergence formed by 

 the incident rays shall be greater, and the lens has no longer the p'ower 

 of rendering the transmitted rays parallel. They will only issue from 

 it in lines less divergent than before. 



The human eye is itself a lens which casts diminished and inverted 

 images of observed objects on the retina, the membrane which is alone 

 sensitive to the effects of light. To the distinct perception of small 

 objects brought near to the eye, there is consequently a limit pre- 

 scribed, since, through an undue approximation of the objects, the 

 image into which the eye collects the rays proceeding from them, falls 

 behind the retina. The distance between the object and the eye, from 

 which this effect results, is different, according to the visual peculiari- 

 ties of observers. Very near-sighted persons are able to see objects 

 distinctly at three inches distance ; consequently they see more of the 

 details or single points of an object than the far-sighted ; for it is alto- 

 gether essential to the distinctness of any object that its image should 

 have a certain extension on the retina. But this extension will be the 

 greater, the wider the angle under which the rays proceeding from its 

 several points strike upon the eye. 



If we interpose now between a very near object and the eye a convex 

 lens, the rays from the object pass through the lens into the eye in 

 parallel or slightly divergent lines, provided the object be at the dis- 

 tance of the focus of the lens or somewhat nearer. The eye is now in 

 a position to cast a well-defined image from the closely approximated 

 object on the retina. The object, which in the case of the near-sighted 

 must be situated nearer the lens, is seen, as if it were no further removed 

 from the eye than the distance from the object to the central point of 

 the lens: being thus seen under a wider angle of vision, it will of 

 course appear larger. 



The magnifying power of a lens depends, on the one hand, on the 

 refractive properties of the substance of which it is composed. On the 

 other hand, a lens will magnify the more strongly, the smaller the 

 radius of the sphere, of which the curved sides represent points of the 

 surface. In a sphere of crown glass the focus is distant from the sur- 

 face about the fourth part of the diameter; in a double and equally 

 convex lens, about the length of the semi-diameter of the sphere of 

 which one of the sides of the lens represents a part. Such a lens, whose 

 curvatures correspond to sections of a sphere of one inch diameter, 

 magnifies, to a sound eye, about eight times. Were it of diamond, it 

 would magnify twenty-one times, since the refractive power of the 

 diamond is two and a half times greater. 



There is no practical difficulty in providing lenses of a spherical 



