20 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



distinctly at a distance of two or three inches, has the same power 

 in his eye alone by reason of its greater convexity, as that which 

 the person of ordinary vision gains by the assistance of a convex 

 lens which shall enable him to see at the same distance with equal 

 distinctness. It is evident, therefore, that the magnifying power 

 of a single lens, depending as it does upon the proportion between 

 the distance at which it renders the object visible, and the nearest 

 distance of unaided distinct vision, must be different to different 

 eyes. It is usually estimated, however, by finding how many times 

 the focal length of the lens is contained in ten inches ; since, in 

 order to render the rays from the object nearly parallel, it must be 

 placed nearly in the focus of the lens (Fig. 3) ; and the picture is 

 referred by the mind to an object at the ordinary distance. Thus, 

 if the focal length of a lens be one inch, its magnifying power for 

 each dimension will be 10 times, and consequently 100 superficial ; 

 if its focal distance be only one-tenth of an inch, its magnifying 

 power will be 100 linear, or 10,000 superficial. The use of the 

 convex lens has the further advantage of bringing to the eye a 

 much greater amount of light than would have entered the pupil 

 from the enlarged object at the ordinary distance, provided its own 

 diameter be greater than that of the pupil ; but this can only be 

 the case when its magnifying power is low. 



18. It is obviously desirable, especially when Lenses of very high 

 magnifying power are being employed, that their aperture should 

 be as large as possible ; since the light issuing from a minute 

 object has then to be diffused over a large picture, and will be pro- 

 portionally diminished in intensity. But the shorter the focus, the 

 less must be the diameter of the sphere of which the lens forms a 

 part ; and unless the aperture be proportionally diminished, the 

 Spherical and Chromatic aberrations will interfere so much with 

 the distinctness of the picture, that the advantages which might 

 be anticipated from the use of such lenses will be also negatived. 

 Nevertheless, the Simple Microscope has been an instrument of 

 extreme value in anatomical research, owing to its freedom from 

 those errors to which the Compound Microscope, as originally con- 

 structed, was necessarily subject ; the greater certainty of its indi- 

 cations being evident from the fact, that the eye of the observer re- 

 ceives the rays sent forth by the object itself, instead of those which 

 proceed from an image of that object. — A history of the means em- 

 ployed by different individuals for procuring Lenses of extremely 

 short focus, though possessing much interest in itself, would be 

 misplaced here ; since recent improvements, as will presently be 

 shown, have superseded the necessity of all these. It may be 

 stated, however, that Leeuwenhoeck, De la Torre, and others 

 among the older Microscopists, made great use of small globules 

 procured by fusion of threads or particles of glass. The most 

 important suggestion for the improvement of the Simple micro- 

 scope composed of a single lens, proceeded some years ago from 



