6o 
PRACTICAL PHOTO-MICROGRAPHY. 
our vision, we tax our eyes less, we have less of the 
blur caused by the planes lying behind and in front of 
each other, and we see more of the surroundings of the 
particular detail under observation, or we have a larger 
“ field ” of view at one time. 
The photo-micrographer, then, and, indeed, every 
microscopist, is urged to use low power glasses when 
possible, and as high numerical aperture as is con¬ 
sistent with general good quality in the objective. It 
is not proposed here to discuss the exact meaning of 
the term “ numerical aperture ” ; it must suffice to sav 
that it not only represents a mathematical fact, but is 
an efficient method of comparing the effective apertures 
of dry and immersion lenses. The highest possible 
“ N.A.” in air is i ; in water, 1.33 ; in cedar oil, 
about 1.52. 
If an object is mounted in the usual way between 
two lamince of glass, and if that object be examined 
with a “dry” objective, air being between the cover- 
glass and the objective, some light rays are reflected 
back from the under-surface of the cover-glass, and do 
not emerge from the preparation at all. Other pencils 
coming out of the denser medium (glass) into the rarer 
medium (air) are refracted away from the perpendicular 
to such an extent that they never reach the objective, 
and so are lost. But if for air we substitute the denser 
medium (water), or the still denser medium (oil), the 
rays are less and less refracted on emergence, and, in 
fact, reach the lens instead of passing into space. In 
the words of Mr. E. M. Nelson, our valued instructor 
