Journal, Sept L110. Royal Microscopical Society. 123 
sure that the structure is there, that it is not an optical deception, 
and that a fine instrument, a careful manipulation, and a good sight 
will not fail to make it out. I may say here that I prefer for such 
an examination a 1th or a }th to a 4th or a yyth, for the reasons 
we are now beginning to understand, wz. that we can, with the 
lower objectives, “take cognizance of more planes ;” and it must not 
be forgotten that the ends of the P. angulatum and quadratum are 
curved or twisted. Also I may observe that, working as I always 
do by preference with a rectangular prism instead of a mirror, and 
consequently with the light directly in front of the microscope, I 
find that the markings I have indicated are considerably more 
distinct when the valve under examination is placed vertically or 
nearly so. 
I trust the Society will not consider these details trivial, for 
independently of making out more perfectly the structure of an 
object which has always been deservedly a “ pet” with microscopists, 
we are beginning to recognize that we are in a transition state with 
regard to our knowledge of the true structure of hundreds of 
objects which were formerly misunderstood, mainly on account of 
the material in which it was thought necessary to mount them, 
greater attention having been paid of late to quality rather than to 
quantity of light for illumination, and the blaze, and the glare, and 
the “pretty” but false appearances produced by Canada-balsamed 
objects having been relegated to the class of observers who look on 
the microscope as a sort of peep-show, instead of a most valuable 
instrument for scientific research, and towards the improvement of 
which I am glad to feel so many minds are now bent, each humbly 
seeking for truth, and none daring to claim even an approach to 
perfection. 
EXPLANATION OF PLATE LIX. 
Fic. 1. Extremity of P. angulatum (reduced } from original drawing). 
2. Variation of rectangular marking on P. angulatum (reduced } from 
original drawing). 
3. P. quadratum, drawn with camera lucida, to show relative extent of 
oblique and rectangular markings (reduced 4). 
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