+ 
230 Dr. North on the Angle of Aperture in Microscopes. 
Again, if we bring into focus the under side, say, of the circle, we 
may Notice a slight mistiness about its general outline, which — 
ey % 
may be removed by entting off half the objective front lens, or of — 
the eye lens. But on more careful examination, we shall perceive 
that we are bringing no one point into perfectly accurate focus, 
and at the same time concentrating our attention upon it. If 
these two things be attended to, all mistiness, thickening, and 
uncértainty, will disappear from such objectives as this gentleman 
employed. Such facts are no real obstacles to microscopic accu- 
racy. ‘They occur equally in unaided vision, whether of small 
or of large objects. If possible, we hold a solid body—a crystal 
for instance—in the hand, and turn it about, yet each change of 
position projects a different geometrical figure. The laws of 
vision are the same with a microscope as without, and no perfec- 
tion of instruments can supersede the necessity of comparin 
different views and of arriving at a knowledge of the third dimen- 
sion in space, through operations of the mind. . 
In some cases a restricted focal distance is both convenient and 
advantageous: one is when we look through a set of markings. 
The Stauroneis pulchella of Rev. Wm. Smith, is described by 
him: “striae very distinct, 30 in 001% punctate; puncta hexag- 
onal.” In his introduction, Mr. Smith remarks: “ ‘The exper- 
ments and authority of Professor Bailey place the existence of an 
internal membrane (in the Diatomacez) beyond all doubt.”—* Tn 
Stauroneis pulchella the membrane in question possesses a0 anu 
sual degree of firmness, the siliceous valves, after a slight mace 
ration in acid, may be seen to fall away from the internal mem- 
Lieberkuhn, and having (to quote the term a plied by the Jury 
of the Great Exhibition) the “exquisite” correction of St 
and Beck, they show by an achromatic condenser, the hex onal 
puncta in three sets of lines, yet obscured in definition, and con- 
fused by the mingling of what seem like shadow 
s. 
Intense illumination of a suitable obliquity and combi “8 
nd ruu- 
of 
the ;*; of 100°, the 1 of still larger angle, and the $ of 1* sh 
128°, these two distinct sets of markings may be seen Wl! 
either perpendicular or oblique light, simply by carrying the be ss 
distance from the one to the other, and without changing the} nf 
mination. ‘The upper strie are at the distance quoted; the B® 
