MICROSCOPE. 



337 



for the purpose of correcting the defects inci- 

 dental to it. 



In order to gain a clear notion of the mode 

 in which a single lens serves to magnify minute 

 objects, it is necessary to revert to the pheno- 

 mena of ordinary vision. An eye free from 

 any defect lias a considerable power of ad- 

 justing itself in such a manner as to gain a 

 distinct view of objects placed at extremely 

 varying distances ; but the image formed upon 

 the retina will of course vary in size with the 

 distance of the object ; and the amount of 

 detail perceptible in it will follow the same 

 proportion. To ordinary eyes, however, there 

 is a limit within which no distinct image can 

 be formed, on account of the too great diver- 

 gence of the rays of the different pencils which 

 then enter the eye ; since the eye is usually 

 adapted to receive and bring to a focus rays 

 which are parallel or slightly divergent. This 

 limit is variously stated at from five to ten 

 inches ; we are inclined to think from our own 

 observations, that the latter estimate is nearest 

 the truth ; that is, although a person with ordi- 

 nary vision may see an object much nearer to 

 his eye, he will see little if any more of its 

 details, since what is gained in size will be 

 lost in distinctness. Now the utility of a con- 

 vex lens interposed between a near object and 

 the eye consists in its reducing the divergence 

 of the rays forming the several pencils which 

 issue from it ; so that they enter the eye in a 

 state of moderate divergence, as if they had 

 issued from an object beyond the nearest limit 

 of distinct vision ; and a well-defined picture 

 is consequently formed upon the retina. But 

 not only is the course of the several rays in 



Fig. 159. 



Diagram illustrating the use of the Simple Microscope. 



each pencil altered as regards the rest by this 

 refracting process, but the course of the pencils 

 themselves is changed, so that they enter the 

 eye under an angle correspondent with that at 

 which they would have arrived from a much 

 larger object situated at a greater distance. 

 The picture formed upon the retina, therefore, 

 corresponds in all respects with one which would 

 have been made by the same object, greatly 

 increased in its dimensions, and viewed at the 

 smallest ordinary distance of distinct vision. 

 A short-sighted person, however, who can see 

 objects distinctly at a distance of three or four 

 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 

 VOL. in. 



assistance of a convex lens which shall enable 

 him to see at the same distance with equal 

 distinctness. It is evident, there lore, 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 ordinarily 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 very nearly in the focus of 

 the lens; and the picture is referred by the 

 mind to an object at ten inches 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 magnifying power will be a 

 hundred linear or ten thousand superficial. 

 The use of the convex lens has the further ad- 

 vantage 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. It is obviously neces- 

 sary, especially when lenses of very high mag- 

 nifying 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 dia- 

 meter of the sphere of which the lens forms a 

 part; and unless the aperture be proportionally 

 diminished, the spherical and chromatic aber- 

 rations will interfere so much with the distinct- 

 ness of the picture, that the advantages which 

 might be anticipated from the use of such 

 lenses will be almost negatived. Nevertheless, 

 the simple microscope has always been an in- 

 strument of extreme value in anatomical re- 

 search, owing to its freedom from those errors 

 to which, as will hereafter appear, the com- 

 pound microscope is subject ; and the greater 

 certainty of its indications is evident at once 

 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 em- 

 ployed by different individuals, for procuring 

 lenses of extremely short focus, though pos- 

 sessing much interest in itself, would be mis- 

 placed here; since recent improvements, as 

 will presently be shown, have superseded the 

 necessity of all these. It may, however, be 

 stated 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 im- 

 portant suggestion for the improvement of the 

 simple microscope composed of a single lens 

 proceeded some years ago from Dr. Brewster, 

 who proposed to substitute diamond, sapphire, 

 garnet, and other precious stones of high re- 

 fractive power, for glass, as the material of 

 single lenses. A lens of much longer ratlins 

 of curvature might thus be employed to gam 



