230 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
condenser, the edge of the flame being in focus, it will be noticed|that 
the illuminated portion of the back lens will be oval and pointed instead 
of circular. Also that when the condenser is racked up, although the 
exterior shape of the illuminated portion will become more circular, 
two dark patches will appear on either side of the centre, showing the 
operation of the spherical aberration of the condenser. If under these 
circumstances the lenses are separated by means of the collar adjust- 
ment, the black spots will be closed up, and a circular and evenly 
illuminated disc of illumination of a larger size will be secured. 
Here we have a distinct gain, in consequence of which we ought to 
be able to see more than we have hitherto done. There is an object in 
my cabinet that has engaged my attention for nearly a quarter of a 
century, viz. the well-known diatom Navicula major . If its “principal 
view ” be examined two vertical stripes will be seen running down the 
centre of the hoop (fig. 33a). In the early days of my microscopical 
Fig. 32. 
work I happened to be looking at this diatom on a dark ground, using 
a Gillett’s condenser with a circular stop, when I was struck by tho 
blue sheen given out by these two stripes ; knowing that this predicated 
unresolved structure, I attacked it with a new water-immersion 1/8, a 
lens which Messrs. Powell and Lealand had recently brought out. The 
illumination was obtained by the same Gillett’s condenser with a moon- 
shaped stop, and the stripes were resolved into strias without difficulty ; 
subsequently these striae were counted, and found to be 60,000 per inch 
(fig. 336), and also they were resolved by an ordinary 1/4 in. of N.A. *74. 
Now, as it is highly probable that there are no such things as striae 
on any of the Diatomaceae, the structure under discussion probably 
consists of rows of very minute perforations, and as it has been my 
endeavour to resolve these perforations, every new improvement in 
microscopical technique has been tried on this diatom, but hitherto 
without success. At last, however, the condenser with the collar cor- 
rection has achieved the feat by means of its enlarged aplanatic solid 
cone (fig. 33c). This is a very good diatom, to work on, because there 
