ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
105 
is projected by means of a narrow axial illuminating pencil.” Now, I 
take it that I have proved to you this evening one thing at least, that 
there is rational ground and experimental proof for the expectation 
that this mixture does come very much nearer to a strictly correct 
projection of the object than that image which is projected by means of 
a narrow-angled illuminating pencil. Finally, I am of opinion that a 
correct understanding of diffractional effects will, more than anything 
else, tend to produce in the minds of microscopists a true appreciation 
of the importance of the achromatic condenser.” 
Apparatus for the rapid change from parallel to convergent 
polarized light in connection with the Microscope.* — Dr. E. A. 
Wtilfing gives a description of an apparatus invented by himself, 
Fig. 4. 
intended to' save time in studying mineral or rock sections by polarized 
light. A plate S (fig. 4), sliding in a groove beneath the stage of the 
* Neues Jalirb. f. Mineralogie, 1889, ii. p. 199-202. 
