Ca;pacity of the Microscope. By Prof. EelmJioIfz. 29 
So far, our demonstration shows that the relation between bright- 
ness of image and ampHfication is entirely independent of any- 
particular construction of the instrument, provided only that it gives 
well-defined images. An increase of amplification would only be 
possible, therefore, when a more intense illumination, e. g. direct 
sunlight were employed, as indeed Listing had in view in the 
methods proposed by him for obtaining enormous amplifications. 
But here other difficulties present themselves, which arise from the 
very slight divergence angle of the emerging rays, as appears in all 
cases of high amplification from the conditions of the equation 
representing the course of rays that enter an objective with wide 
divergence angle. 
The first difficulty is, that shadows of entoptic objects throng the 
field more densely as the area of this field at the eye spot (ocular 
image of the objective) becomes smaller. The retina is illuminated 
from this area as if it were the source of hght from which pro- 
ceeded all the rays that enter the eye. This area is at the same 
time the basis of the collective pencils which belong to the several 
points of the object, and of its image on the retina, and its diameter, 
as before shown, varies in inverse proportion of the amplification. 
But the very conditions which must be fulfilled in order to obtain 
sharply defined shadows of objects within the eye are exactly what 
occur here, namely, that a strong light should enter the eye from a 
relatively small surface. 
Whoever has, at any time, attempted to illumine the field of the 
microscope with direct sunlight, when employing a high amplifica- 
tion, will remember the peculiar spotty appearance of the field so 
obtained. Some of these spots remain fixed in the field, but others 
move with the motion of the eye. The first class of spots is due to 
dirt particles or imperfect polish of the ocular lenses ; the second 
arises from shades caused by intervening opacities in the tissues of 
the eye — conjunctiva, cornea, crystalline lens, or vitreous humour.* 
This method has even been used to discover their existence, and is, 
in truth, a very suitable one. In proportion, however, as entoptic 
objects become more noticeable, will a greater number of finer 
details of microscope objects become obscured. 
A second and inevitable disadvantage arising from the narrow 
divergence angle of the emerging rays shows itself in the occurrence 
of diffraction phenomena, whereby the outlines of visible objects 
are efiaced, and at the same time doubled or further multiplied. 
We have to deal here chiefly with difiraction phenomena as they 
appear when we look through a minute circular opening. A bright 
point of hght (reflexion of sun on the bulb of a thermometer) 
viewed through a pin-point hole pierced in a card appears as a 
* But mainly from the retinal vessels, as shown by Heinrich Miiller, vide 
Wurzburg Verhandlungen,' vol. v. p. 411. — H. E. F. 
