The Penetrating Power of the Microscope. By E. M. Nelson. 335 
the object. It would be therefore absurd in the extreme to call this 
penetration with strained accommodation the penetrating power of the 
Microscope. 
Let us now see bow the penetration with the strained accommo- 
dation of normal sight compares with that laid down by Prof. Abbe. 
Abbe’s myopic accommodation (150 mm. to 300 mm.), power 10, 
N.A. ’08 and a balsam mount, would give 3-125 mm. for accommo- 
dation depth, and -682 for focal depth, or a total of 3-807 mm., 
which is • 807 mm. more than that of strained normal accommodation ; 
but as the unstrained accommodating distance for normal sight, viz. 
8 in. to oo, is greater than that of Abbe’s myopic accommodation of 
150 mm. to 300 mm., the penetration should also be greater, but 
this experiment shows that it is less. 
So far we have been experimenting with normal sight only. If we 
try myopic sight we shall find that penetration will be greater than 
that for normal, instead of less, as Abbe’s theory requires. It is very 
instructive to notice the treatment of the microphotograph under a low 
power by myopic sight. A myopic eye is unable to mention any 
definite focus. Myopic microscopists are always playing on their 
accommodating powers, and consequently are unable to find a precise 
focus with a low power. For this reason they are able to penetrate 
without straining their accommodation to a greater depth than persons 
with normal sight. 
My theory of microscopic penetration due to accommodation is, 
that with normal vision the eye does not accommodate at its nearest 
but at its easiest distance of vision, and that in some unexplained way 
the accommodating power of the normal eye is to a certain extent 
paralysed when using a Microscope. Hence, there is only a very 
limited amount of penetration without strain on either side of the 
easiest accommodating distance. Moreover, a normal eye when using 
a Microscope and straining its accommodation to the utmost, is not 
able to compass so large a range of accommodation as it ordinarily 
does without any strain when not using a Microscope. A myopic 
eye, on the other hand, is continually straining its accommodation, 
and it is on this account that the paralysing effect of the Microscope 
is not so much felt. 
But from experiments made with myopic persons, I should be 
inclined to think that all the paralysing effect is not removed, and 
consequently the penetration is not so great as Abbe’s table would 
lead one to imagine. 
It is interesting to note in passing, that the alteration in the mag- 
nifying power can be distinctly seen when performing the straining 
experiments with the microphotograph. 
We will now pass on to the investigation of Prof. Abbe’s tables, 
which are very simple, and are based on elementary principles, with 
which every microscopist is acquainted. 
The difficulty about the whole matter, and for which I can find no 
