38 Transactions of the A^ierican Institute. 



may see it all. As the object recedes the image diminishes. The 

 light from it at the same time grows faint. Owing to these causes 

 combined, there is a limit of distance, beyond which we fail to per- 

 ceive objects which themselves have an appreciable magnitude. As 

 an object approaches the eye, its image on the retina grows larger, 

 and points in it, which, at a greater distance were below the mini- 

 m.um visibile, become perceptible. But there is a limit within which, 

 if we bring it, it ceases to be distinctly seen, because the eye has no 

 longer power to maintain a clear image on the retina. This is called 

 the limit of distant vision. Any contrivance which will enable the 

 eye to form such a clear image without requiring the object to be 

 carried further oif, will show the object uimaturally large. If we 

 look at the letters of a printed page through a puncture made' by a 

 needle in a card, this magnifying effect may be observed. The con- 

 fusion of the image is very nearly removed by the effect of the punc- 

 ture in cutting oft' all but the central rays of each pencil from the 

 object. 



A convex glass introduced between the object and the eye, the 

 object being nearer than the principal focus of the glass, produces a 

 similar eftect, but more perfectly. The glass does not enlarge the 

 image on the retina, but it aids the eye in clearing it up. What the 

 eye in this case sees, is the image formed in the negative focus of the 

 glass, conjugate to the focus occupied by the object. This negative 

 focus must, of course, be at least as far oft" from the eye as the natural 

 limit of distinct vision. This limit is not the same for all eyes. It 

 varies with individuals, from seven or eight inches to eleven or twelve. 

 For all purposes of computation, in estimating the powers of the 

 microscope, it is taken invariably at ten inches. The magnifying 

 power of a lens, is the apparent increase expressed in numbers, which 

 it gives to the diameter of the image of an object above that of the 

 object itself as seen at the limit of distinct vision. It may be deter- 

 mined by observing the object through the glass with one eye, while 

 with the other a rule is observed held before it at the distance of ten 

 inches. The image will be seen superposed upon the rule, and the 

 enlargement may be directly read oft* Very large magniiying powers 

 have been obtained with single lenses, Tlie early microscopists, Leu- 

 wenhoek, Swammerdam, Hooke, Lieberkuhn, and others, used such 

 exclusively. They formed them, in many instances, by melting 

 minute fragments of glass, which, while fluid, assumes the spherical 

 ehajje, and on cooling may be used without grinding. But to use 



