576 EHRENBERG ON THE POWER OF VISION OF THE EYE 
They plainly demonstrate an unfathomableness of organic life in the 
direction of the smallest conceivable space ; and if the word infinity be 
too much for what we know at present, let the word unfathomableness, 
which I have purposely employed, avert from me the reproach of exag- 
geration, and establish the point of view which the physical, chemical, 
and physiological inquiries of our days, should they be rendered fruitful 
by new powers, have to take, and what deviations they have to avoid. 
III. An attempt to form a judgement respecting the Power of Vision of 
the Eye and the Ultimate Power of the Microscope. 
I will now connect with the above paper, for its illustration and con- 
firmation, a few considerations on the power of the human eye, and on 
the confidence and the hopes which we may found upon microscopical 
observations and optical instruments. Up to the present moment, so 
far as I know, we have never been able to fix a constant measure for 
the ultimate possible power of the microscope. M. Amici, in a letter to 
the Baron von Zach in the year 1824, (Ferrusac, Bullet. des Sc. Ma- 
them., p- 221,) has calculated the limit of vision according to the 
power of the eye, and stated that a space of ;4; of an inch becomes im- 
perceptible to the naked eye at the distance of 28 feet. Lately the 
angles of vision for the different colours have been calculated by M. 
Plateau, but I have no:knowledge of any result from similar observa- 
tions having reference to the microscope. I will endeavour to lay down, 
without any pretensions, my investigations respecting the limit of vision 
by the microscope made in a different way ; and I shall rejoice if they 
make an addition to this branch of our knowledge and are not wholly 
without utility. 
In the numerous opportunities which I have had of watching persons 
eager for knowledge who were desirous of acquainting themselves by 
personal observation at my house with the wonderful structure of the 
infusoria, I found to my astonishment the difference of the power of 
vision of individuals by far more nearly coincident than I had expected, 
and than itisgenerally stated tobe. When once I had placed the delicate 
object in the right point of vision of the instrument, or directed the 
attention of the naked eye to a very minute object, fifteen or twenty 
persons, to whom I often showed these things at the same time, saw 
completely alike and with the same clearness what I myself saw: they 
very seldom took another, and then but little different, distance of the 
object from the eye, according to what they required. In order to be | 
quite certain that I was not deceived by the politeness or shame of those 
who might not willingly say they had seen nothing, I have often desired 
the observers to delineate the objects seen, or minutely to describe 
them; by which I learnt with certainty that they saw the object exactly 
the same and quite as distinctly as I myself had seen it, and almost — 
