224 Dr. North on the Angle of Aperture in Microscopes. 
Yet it is important to keep in mind that there ¢s a limit which 
no illumination or enlargement of aperture will overcome without 
increase of magnifying power. It may lead to serious error to be 
accustomed to strain the eye with a low eye-piece or objective, 
when a more extended amplification of image ought to. be em- 
ployed for more decidedly separating lines or points. M. Robin 
argues for employing high powers in anatomical investigations, 
with an earnestness which proves a then prevalent error. Prob- 
ably such mistakes have contributed to establish an impression of 
there being often a danger of using too much light. When en- 
deavoring to look into minute structure with a low or medium 
power, when a high one is what is needed, one will unconsciously 
strain and prolong his attention, until, if the light be strong, 
-he brings on confusion, dazzling and pain, which a weaker light 
may alleviate, and at the same time be sufficient for what the 
of lines, which is so often noticed. Although not well defined, 
those now in question are seen in focus, and although thickened 
and badly defined, are yet plainly visible and easy to count 10 
micrometrically measuring their distances.. Among other expe! 
iments in proof, the following is selected: The ribs of Pinnula- 
ria major’ were viewed with a low objective and intense !ampP 
light, (sunlight is still better,) at an obliquity, very near the 
ich produces a dark back-ground. The ribs pia 
strongly visible, though not entirely across the semi-breadth of t 
valve. Being easily counted, the micrometer gave their or 
saz and ;,';5 of an inch in different cireumstanees, with su™ 
cient accuracy for the experiment. But when with the ame 
light and illumination, a 4th objective was employed, the! 
-were visible in their whole length, and with a perfect definitio™ 
Their distance was, in the specimens examined, beyond all Kate 
for doubt, found to be very accurately equal to only ;z307 ith 
Two acute microscopists and practiced observers, one of them 4) 
able microscope maker, witnessed, and themselves repeated t nt 
demonstration, and confirmed its reliability. The expet™®! 
