WELCKER, ON MICROSCOPIC MEASUREMENTS. 245 
known, it may be deduced whether the apparent thickness of 
the body under examination should be corrected to the 
refractive power of aqueous media (100—142), fluid fat 
(100—150), or the most strongly refractive substances (100 
—160—180). If a substance (e.g. glass) lying in Canada 
balsam exhibit the bright spot when the tube is elevated, and 
in oil of aniseed when it is depressed, it may at once be con- 
cluded that the refractive power of the body lies between 
154 and 158.* 
In the case of a spherical or cylindrical body, it is obvious 
that the determination of the horizontal diameter by com- 
mon linear measurement will be preferred to the estimation 
of the vertical thickness. But the question is different when 
it is doubtful whether the figure be really spherical or cylin- 
drical, or not rather of some other rounded form.t In the 
determination of the vertical diameter, which (if the object 
does not admit of rotation) may in cases of this kind be 
desirable, it is assumed that the refractive power of the 
surrounding medium is known. 
In the experiments above detailed, in which the object was 
a horizontal plate, the objective, when focussed upon the two 
surfaces, travelled in one and the same direction, dependent 
upon the thickness and refractive power of the particular 
object, and communicated one and the same thickness, whe- 
ther the object were placed in air and exhibited a great 
degree of briliiancy, or in oil and wholly without any. If 
the object, however, be a sphere (and precisely analogous 
phenomena are exhibited im other bodies having rounded 
surfaces) the microscope will afford a view of the upper and 
of the lower surfaces only when the refractive power of the 
object and that of the surrounding medium are equal or 
nearly so. When the object possesses a considerably greater 
refractive power than the surrounding medium, the focussing 
of the objective will afford, not the “ optical transverse dia- 
* Conclusions of this kind are inadmissible when the surrounding fluid 
exerts any kind of chemical or physical influence upon the object of such a 
kind as may alter its refractive power. Thus, in glycerine the striped mus- 
cular fibre shows brightness on the elevation of the tube, because its refrac- 
tive power is increased (probably by imbibition of water), and it thus appears 
to possess a stronger refractive power than glycerine. But the refractive 
power of unaltered muscular fibre is by no means higher than that of gly- 
cerine, but far less, being little greater than that of blood-serum. 
+ With respect to this, it is well known that our vision may be deceived 
in very many ways by refraction. Thus, in the case of perfectly cylindrical 
objects, the appearance of flattening is always produced when the refractive 
power of the surrounding medium is but little less than that of the inclosed 
object. Thus a cylindrical thread of glass in Canada balsam appears’ as a 
flattened band-like streak of little brilliancy. 
