128 THE AMERICAN MONTHLY (May 
the smallest possible beam of truly axial illumiaation,the 
number of lines to the inch capable of being resolved — 
« X N. A. where 2 is the number of wave-lengths to the 
inch of the light actually used. Putting this into actual 
figures, seeing that there are about 47,500 X ‘95 gave 465,- 
125 lines to the inch as the theoretical limit—a long way 
off 100,000. In other words, the lines must not be closer 
than 1-45125 of an inch. But with oblique light this for- 
mula was doubled, and became 22°95, or 90,250 to the 
inch, or 1-90250 of an inchapart. It was evident, then, 
that Mr. Merlin could not have seen lines 1-100000 or even 
1-90000 of an inch, apart without oblique light, using only 
a 5-6ths cone of axial illumination ; and this justifies the 
original remark that his specimen must have been a coarse- 
ly marked one. It was theoretically possible that the au- 
thor might possess a photographic eye, so to speak: one 
that received impressions in the violet-blue ray as well as 
ordinary individuals did in the yellow-green or so-called 
‘‘visual ray,” but he had never heard of such a case. 
As theformulaalready given applied toany ray,itshould 
be possible to photograph on a plate what cannot be seen 
with the eye. The 100,000 lines to the inch could only be 
seen most faintly with the ‘95 objective, but inasmuch as 
the wave-length of photographic light was about 1-62500 
of an inch, twice that X ‘95 gave a photographic limit of 
a little over 100,000 lines to the inch. His son, and him- 
self had tried to do this. As they could focus the lines 
on the ground glass screen of the camera, they had to 
make trial and error exposures, and failed several times, 
but at last succeeded in just showing the lines. 
Seeing things with a direct solid cone was no doubt 
very much better and more to be relied upon than seeing 
them by oblique light. An object with large markings 
well seen by direct light appeared simply gray with ob- 
lique light. Ifthe lens.employed was a fine one and the 
lines were very fine they could be seen with oblique light, 
