AND THE ULTIMATE POWER OF THE MICROSCOPE. 577 
always without its being in the first place necessary to change the po- 
sition of the microscope. 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 opinicn that 
myopical people could see more and more distinctly than other persons 
is quite unfounded. The result of my experience is twofold : 
1. There seems to be anormal 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- 
stinetly, 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 was necessary to have the object approached to or removed 
from their eye. 
2. The smallest square magnitude attainable in general by the hu- 
man eye in its natural state amounts for white on black ground, as well 
as for black on white or light-coloured ground, to 3, of a Parisian line 
in diameter. It is possible by the greatest condensation of light and 
excitement of the attention to recognise magnitudes between =) and 
aq Of a line, but without sharpness and certainty*. 
This is the limit of the power of the natural human eye for coloured 
bodies, of which everybody 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 =), 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 ~, would not be worth while. The next proportions worth 
notice were ;', or =, of a line, and I have never been able to make any trial 
whether it could be seen by any one. 
