D 
PRACTICAL PHOTO-MICROGRAPHY. 
converse is the most difficult problem the optician has 
to face. Lenses are frequently quoted as having a 
high angle of aperture and a low magnifying power; 
when put to the test they will be found to be of higher 
power than stated, or lower aperture. 
“Penetration” is an imaginary quality supposed to 
represent the power of giving at the same time a sharp 
focus on two separate planes at right angles to the 
optical axis, which is plainly absurd. A lens may be 
of such nature that no plane is thoroughly sharply 
focussed, and so the eye may be deceived into an im¬ 
pression of sharpness on several planes, simply because 
it finds no sharpness to compare with blur. Yet this 
want of sharpness on any plane, and impression of 
sharpness on several planes, may have its value, how¬ 
ever small, in certain branches of research, where only 
general appearances are the object of study. But in 
photo-micrography all blur is fatal to success. Com¬ 
promises in sharpness between one plane and another 
will not do, we must have the greatest possible sharp¬ 
ness on the critical plane at least ; and further, this is 
the best plan for getting crisp definition as nearly as 
possible over all, as will be found by anyone who makes 
carelul comparison between wide and narrow angled 
lenses at the same magnification. The blurring, or 
“ confusion,” due to depth of subject, increases as the 
aperture increases, but it increases as the square of the 
magnification. 
As it is of the utmost importance that this should be 
thoroughly understood, we put it in the words of Dal- 
