242 WELCKER, ON MICROSCOPIC MEASUREMENTS. 
estimation of it in proportion to the degree in which the 
refractive power of the substance exceeded that of the air.* 
I give, in the first place, a very generally applicable method 
of determining the refractive power, or of the “ apparent 
height” of microscopic objects, together with some numbers 
by which, from the “apparent height” thus determined, the 
“true height” of the object may be calculated. 
If the microscope, like the instruments of Kellner, Belthle, 
Oberhauser, &c., is furnished with a fine-adjustment screw, 
having a horizontal head, this will be readily made into a 
micrometer, by marking off each fifth or tenth groove on its 
edge by a fine line, to be numbered accordingly. The index 
may be formed by a small point suspended from any fixed 
part of the microscope immediately above the screw-head, and 
just touching it. Lastly, the value of a turn of the screw is 
ascertained, as well as of parts of a turn.t 
In my microscope, made by Kellner, whose screw-head is 
accurately divided at the edge into 205 parts, I obtained the 
following figures : 
30 turns of the screw depress the tube about 11‘Omm. 
1 turn consequently = 0°3667mm. 
1 notch at the edge (=z3, turn) = 0°0018mm. ” 
By means of the screw of this microscope I then estimated 
the refractive power of numerous substances, selecting in 
fact, for the purpose, the most important. fluids and trans- 
parent tissues of the animal body, as well as some of the 
substances used in the examination and preservation of mi- 
croscopic objects. 
Two narrow strips of glass, not quite 1mm. thick, were 
glued upon the object-bearer, a few millimetres apart, and 
upon these was laid thin covering glass. The two glass slips 
inclosed between them a stratum of air, whose thickness, 
by accurate measurement, was found to be 0°9873mm. 
On the under surface of the covering glass, as well as on the 
upper surface of the object-bearer, very fine lines were 
scratched with a diamond ; the upper ones in a vertical, and 
the under in a horizontal direction. The accurate micro- 
scopic definition of these two sets of lines required a move- 
ment of the tube corresponding to two turns of the screw 
and 142 notches, in all 552 notches.t When the stratum 
* This wxder-value, according to the author’s investigations, amounts, in 
substances possessing slight refractive power, to about one third, and in 
those of very considerable power to nearly one half of the true value. 
+ It is scarcely necessary, perhaps, to remark that a properly divided 
screw-head accompanies the fine-adjustment of most English microscopes. 
+ In all cases the upper set of lines were first viewed, and then, by a 
