SIMPLE MICROSCOPE. 79 



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. 2) ; 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 ten times, and consequently a hundred super- 

 ficial ; if its focal distance be only one-tenth of an inch, its mag- 

 nifying power will be a hundred linear, or ten thousand superfi- 

 cial. 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 proportionally 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 inter- 

 fere so much with the distinctness of the picture, that the ad- 

 vantages which might be anticipated from the use of such lenses 

 will be almost 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 constructed, was necessarily subject ; 

 the greater certainty of its indications being evident from the 

 fact, that the eye of the observer receives the rays sent forth by 

 the object itself, instead of those which proceed from an image 

 of that object. A detail of the means employed 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, how- 

 ever, 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 Microscope com- 

 posed of a single lens, proceeded some years ago from Dr. 

 Brewster, who proposed to substitute diamond, sapphire, garnet, 



