240 
On the Measurement of the Vertican Tuickness of 
Microscopic Ossects ; and on the DETERMINATION of the 
Cuemicat Propertiss from their REFRACTIVE Powrr. By 
Dr. H. Wetcxer, of Giessen. 
In the ‘ Zeitschrift f. Rat. Medicin,’ N. F., vi, p. 172, and 
vill, p. 241, the author endeavoured to show, that when 
by a methodical raismg or depression of the tube of the 
microscope the relief of microscopic objects has been ascer- 
tained,—the question, especially as respects the more difficult 
class of objects, may be decided, as to whether they are 
hollow or solid, depressed or convex. If a microscopic 
object be of such a size as to allow of its near and distant 
surfaces being each distinctly and separately brought *into 
view by the proper adjustment of the tube of the instrument, 
the determination of this question is usually unattended with 
any difficulty ; the proceeding, in fact, beg one commonly 
practised. The author, however, has endeavoured to show, 
that in the smaller or even in the most minute microscopic 
formations—so small even that they appear only as simple 
dark points—a bright flash or gleam ensues upon the eleva- 
tion of the tube, in those cases that is to say in which the 
bodies in question are convex; and a similar flash on the 
depression of the tube, when they are concave. “I have 
shown,” he says, “that in the former case the convex 
corpuscles act like minute convex lenses, and in the latter 
like concave lenses. Oil-drops contained in water or air, 
albuminous and many other sorts of particles,—in short, all 
such as in regard to their optical property are referable to 
the perfectly spheroidal form, show the brilhant appearance 
on the elevation of the tube; whilst minute air-bubbles, the 
cavities in bone, teeth, &c., containing air or fluid, &e., pre- 
sent the same phenomenon when the tube is lowered. 
Since in by far the greater number of cases the fluids of 
the animal or vegetable organism have less refractive power 
than the solid constituents suspended in them, or which, on. 
the other hand, may lodge the fluids in question in internal 
cavities, it is apparent as a rule that when an object exhibits 
its utmost briliancy on the elevation of the tube, a con- 
vexity must exist; and again, that it must be concave when. 
the brilliancy is witnessed on the depression of the tube. 
But I haye lastly remarked, that for the determination of the: 
relief of a microscopic object it is of essential importance: 
also to regard the refractive property of the surrounding 
medium. For under certain circumstances, even perfectly 
