246 WELCKER, ON MICROSCOPIC MEASUREMEN'S. 
meter,” as it is said, more particularly in such instances, by 
the majority of authors, to do, but only of that part of the 
spherical surface which lies above the equator; the under 
hemisphere exhibiting only very faint images. If the object 
possess less refractive power, or be a hollow sphere, the optical 
focussing reaches only the lower half of the hollow sphere. 
If the distance traversed in the focussing of the objective 
between the apex (or base) and the equator be now mea-~ 
sured, we obtain a reciprocal value according to the quality 
of the surrounding medium. We measure, in fact, the 
apparent depth of a layer of the surrounding medium, which, 
corrected for its true depth, is nearly equivalent to the semi- 
diameter of the sphere. In exactly the same way, in the 
case of rounded objects not of a spherical figure, the focus- 
sing of the objective embraces that part of the surface of the 
object which rises above the greatest horizontal section, 
when that portion of the object refracts the light lke a 
convex lens, and the under portion when it acts like a concave 
lens. In this case the movement of the tube bearing the 
objective corresponds to the apparent thickness of a stratum 
of the surrounding substance, whose true thickness is nearly 
equal to the height of the rotundity to be measured. 
2. The determination of the chemical quality of micro- 
scopic objects from their refractive power. 
The optical relations of objects have hitherto been em- 
ployed only to a very limited extent as a means of diagnosis 
in microscopic researches. Leaving out of account the 
application of the polarizing apparatus, observers have hitherto 
confined themselves almost entirely to speaking of the greater 
or less refractive powers of objects according to the greater or 
less brilliancy of the light proceeding from them. But not 
only is this mode of description in itself vague, but in addi- 
tion, so long as the refractive power is estimated simply 
from the impression made upon our eyes by the object— 
that is to say, from its greater or less brilliancy—our judg- 
ment is exposed to great deceptions. For, as has been shown 
above, it depends wholly and solely upon the refractive 
power of the medium surrounding the body under investiga- 
tion, whether the refractive power of the body can be esti- 
mated from the phenomenon of its brilliancy or not. Thus 
in the case of microscopic fatty particles, the strong bril- 
lianey of the fat affords one of the most useful means of 
recognition; but it must be remembered, that fat injected 
into the canaliculi of a bone, owing to the greater refractive 
power of the bone, does not exhibit its usual brilliancy. 
