AND THE ULTIMATE POWER OF THE MICROSCOPE. 577 



always without its being in the first place necessary to change the po- 

 sition of the mici'oscope. These observations, continued carefully on 

 a great number of persons, with the most various distances of vision, 

 made it seem probable to me that there is a nearly fixed common limit 

 for the power of vision of the unclouded and healthy human eye, which 

 will admit of our forming a conclusion as to the ultimate power of the 

 microscope. Upon this I made various observations, in order to find 

 out how far the variations of myopical and presbyopical eyes pos- 

 sessed an influence on the general expression of that power; and I have 

 convinced myself various times that the not unfrequent opinion that 

 myopical jieople could see more and more distinctly than other persons 

 is quite unfounded. The result of my experience is twofold : 



1 . There seems to be a normal power for the human eye in reference 

 to the seeing of the minutest particles ; and the deviations from it 

 appear to be much more rare than is generally believed. 



We can only speak of those who at some distance or other can see 

 distinctly. Among more than 100 persons that I have observed, there 

 were those who in the general relations of vision could see most di- 

 stinctly, not capable of distinguishing more than I myself saw; and those 

 who represented themselves weak-sighted or long-sighted were in ge- 

 neral capable of seeing in the same degree as I did, only they wanted 

 the object more minutely pointed out ; and besides, in seeing with the 

 naked eye it Avas necessary to have the object approached to or removed 

 from their ej'e. 



2. The smallest sqvxare magnitude attainable in general by the hu- 

 man ej'e in its natural state amounts for white on black ground, as well 

 as for black on white or light-coloured ground, to ^^v of a Parisian line 

 in diameter. It is possible by the greatest condensation of light and 

 excitement of the attention to recognise magnitudes between -J^ and 

 ^ of a line, but without sharjiness and certainty*. 



This is the limit of the power of the natural human eye for coloured 

 bodies, of which everybodj;^ can satisfy himself, as I have done, by 

 strewing on white paper very fine black particles of dust, for instance, 

 of dried ink, water colours, &c., and then taking the smallest of them 

 with a fine point and placing it on a glass micrometer, which at least 

 gives the :^',j of a line. Sun- and lamp-light also allow easily of our ob- 

 serving black particles and the like, with or without a mirror, on the 

 glass micrometer on a light ground. Bodies which are smaller than 

 those mentioned, notwithstanding all attempts, cannot be discerned with 

 the naked eye singly, but may be when placed in a simple straight row. 



• To insist on Vrr would not be worth while. The next proportions worth 

 notifo were ^"o or y,, of a line, and I have never been able to make any trial 

 wlictlier it could be seen by any one. 



