10 RICHARDSON, ON STOPS FOR OBLIQUE [LLUMINATION. 
how far the raphidian character may prove useful in the re- 
vision which this and some of the other orders seem to re- 
quire remains to be decided after a careful extension and 
correction of these observations, especially as regards the 
Flora of the World, by judicious inquirers, who may have the 
requisite materials at their command, and the will to use 
them, for the elucidation of the question of the value of 
raphides and their cells as natural characters in systematic 
botany. Meanwhile it is hoped that the present memoir 
may induce some of our countrymen to study the subject in 
their own Flora. 
Stops recommended for Oxsutque ILtumination with the 
Acuromatic ConpenseR. By B. Wiis Ricwarpson, 
F.R.C.S.1., Surgeon to the Adelaide Hospital, Dublin. 
Tue attempts to resolve the markings of certain diatoms 
with oblique light are frequently attended with considerable 
difficulty, so much so that the management of oblique illu- 
mination requires very great patience to prevent failure. One 
moment the field is too milky, at another the glare is most 
distressing, next the valves are too thick, and at last, after a 
great deal of trouble and strain of vision, a tolerably good 
view is obtained. But, on the other hand, we may not be so 
fortunate, and, notwithstanding all our perseverance, com- 
plete failure in procuring a satisfactory demonstration is often 
experienced. Of course, I assume that the object-glasses are 
properly adjusted, a neglect of which is of itself sufficient to 
interfere with the delineation. 
Not only have I seen the difficulties enumerated expe- 
rienced with the mirror and condenser, but likewise with 
the prism, when used for oblique illumination. And even 
when the diatom markings are sharply brought out with the 
latter, there is often a milkiness of the field and object very 
distressing to the eye of the observer. 
During the course of the summer I devoted a few evenings 
to the making of a variety of stops for my achromatic con- 
denser, hoping that by so doing I might succeed in construct- 
ing stops better adapted for rapid demonstration of markings 
by oblique illumination than the solid discs I had hitherto 
been in the habit of using. 
