1901] MICROSCOPICAL JOURNAL. 127 
absolutely necessary. First, that such objects should be 
sufficiently magnified for the eye to be capable of seeing 
them ; and secondly, that the N.A. of the objective should 
be high enough to render such objects sufficiently resolv- 
ed ; for every one in the room was familiar with the fact 
that mere magnification without sufficient N.A., or “emp- 
ty magnification,” as Professor Abbe called it, was as use- 
less as N.A, without the proper amount of magnification. 
Now with regard to the first condition. It was sup- 
posed thas 1-250 inch represented the minimum distance 
that two objects, whether lines or dots, must be separated 
for the normal human eye to seeand separate them dis- 
tinctly at a distance of ten inches. No lines or dots closer 
than this could be recognized in their individuality. In 
other words, no matter what might be the real distance 
between any two dots or lines on a diatom they must, by 
optical means, be so rendered to the eye, when looking 
down the microscope, that they did not appear closer to- 
gether than 1-250 of an inch. It was more convenient for 
them to be magnified a little more, so as to be separated 
apparently by a greater interval, because in that case 
those whose eyes were not absolutely normal would see 
them better ; but anyhow they must not apparently be 
separated by an interval of less than 1-250 of an inch. 
The lines on Amphipleura pellucida were mostly about 
100,000 to the inch, so to see them with the microscope 
the entire optical arrangement must result in magnify- 
- ing at least 400 diameters, because 400 X 250 = 100,000. 
Now, how did the author obtain his magnification, and 
what was it? He used a @ in. objective and a 27 eyepiece. 
Well, that equalled a magnification of 1620, because the 
initial magnifying power of a % in. was about 60, and 60 
<x 27 = 1620. He had, therefore, plenty of magnifica- 
tion. But what about the N.A.—the second condition ? 
Abbe’s law which was based on mathematical consid- 
erations admiting of no controversy, declared that, with 
