PENETRATION IN OBJECTIVES. 173 
in the microscope is obtained, but the subject has never been 
popular, nor will it be so until an effort is made to show, in abso- 
lute measure, how much penetration can be obtained from any 
given objective. 
To any one who has mastered Professor Abbe’s elaborate 
papers on this subject an explanation will be unnecessary, but to 
many it may be an aid, by freeing the matter from the greater 
portion of its technicalties, and it is hoped will also act as an 
incentive to the study of “penetration” and “aperture” a little 
deeper than is usually the case. 
In making microscopical measurements the most convenient 
unit to adopt is the mcra. This unit is the thousandth part of 
a millimetre, or the ;+,, part of an English inch ; but if a proper 
appreciation is obtained of the magnitude of the mcra it is un- 
necessary, nay, injurious, to hamper one’s mind with the knowledge 
that it is a fraction of either of the above linear standards. 
Let the student of microscopy consider the micra on its own 
merits, as a minute standard of measurement, the magnitude of 
which may be appreciated by the knowledge that the small 
diameter of some species of bacteria is equal to one micra, the 
spores of the mould fungus Penicillium glaucum 3'0 micras, the 
long diameter of tous-les-mois starch about 96 micras, while the 
diameter of many of the Foraminifera reach from 300 to 800 
micras. For brevity the word micra is usually written p. 
Now, Professor Abbe has told us that the penetration of the 
microscope depends principally upon three factors :— 
1. The accommodation of the eye. 
2. The focal depth of the lens itself. 
3. The medium in which the object is immersed. 
So that a practical turn may be given to the subject by elaborating 
the figures given by Professor Abbe, on page 680, vol. 1., series 
i, of the Journal of the Royal Microscopical Society. There is 
little doubt that such a method as this will help the student to 
understand the true value of penetration in by far the best manner ; 
and may possibly give a hint to the photomicrographer as to the 
lenses he must employ when it is desired to photograph solid forms. 
It will be readily understood that when it is desired to photograph 
pyramidal forms the penetration must necessarily enable a well 
defined picture to be taken from apex to base at one focussing ; 
but when the object is of spherical nature the penetration need 
only approximate to little more than half the diameter, unless it is 
wishful to show the uppermost layer of the preparation, as well as 
the slide itself, simultaneously. 
I. The ‘accommodation depth” may vary a little according to 
the sensitiveness of the observer’s eye, and other physiological 
conditions ; but with the same observer the only serious quantity 
