MICROSCOPES. 19 



it ; and at the distance of six or eight inches, its apparent magnitude is 

 so great, that it appears to cover a distant object fifty or one hundred 

 times its size. By thus bringing the coin nearer the eye, we have 

 actually magnified it, or made it apparently larger, though its size 

 remains the same. Again, as a further example of this : if we look at 

 two men of the same height, the one 200 feet, the other 100 feet from 

 us, the former will appear only half the height of the latter, or the 

 angle which the latter subtends to the eye of the observer will be 

 twice that of the former. Hence it becomes evident that the nearer 

 we can bring an object to the eye the larger will it appear. 



9. But let us suppose that the object be distant from us twenty 

 feet, and let a convex lens, whose focal length is five feet, be placed half 

 way between it and the eye, that is to say ten feet from each, then it is 

 plain that the image of the object, as formed by the refracting power of 

 the lens, will be exactly of the same size as the object, and consequently 

 it is not directly magnified by the lens ; but as the image is brought 

 so near to us that the eye can view it at the distance of six inches, its 

 apparent magnitude is increased in the proportion of six inches to 

 twenty feet, or as one to forty, that is forty times. It is, in fact, 

 magnified forty times, merely by bringing an image of it nearer to 

 the eye. 



1 0. But if wehave to examine a very minute object, and in order to 

 render its parts distinguishable we bring it very near to the eye, within 

 an inch or two, for example, it will become very indistinct and confused. 

 This effect is produced by the great divergence of the rays of light 

 from the object, and the power of the chrystalline lens of the eye not 

 being sufficient to collect the rays, whereby an image of the object may 

 be formed on the retina, at the proper distance on the back of the eye. 

 Now if we employ a convex lens, and place it between the object and 

 the eye, the former being in the focus of the glass, the diverging rays 

 from the object will be refracted and rendered parallel by the lens, and 

 thus we shall obtain a distinct and near view of the object. If we 

 place the lens close to the eye and the object, in such a way that the 

 rays which flow from it may receive that precise degree of divergency 

 which they had when the object is placed six inches from the eye, the 

 nearest distance at which we see minute objects distinctly, if the dis- 



