ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 
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the deposition of the metal, h its length, d half the increase in thickness 
(so that r x = r 4- d) x and v x the volume of the deposited metal, then 
v x = Ji x 7r r x 2 — h . 7r r 2 
= h 7r (r -f- d) 2 — h . 7rr 2 
= Ji7r(2rd + d 2 ). 
If s denote the specific gravity and p the weight of metal deposited, 
p = s . v x = 9 . li tt (2 r d + d 2 ). 
Thus, if the specific gravity of the electrically deposited metal and 
the original thickness of the platinum wire are known, the weight of 
metal deposited can be determined by measuring with the micrometer 
the increased thickness (2 d) of the wire. The use of the apparatus for 
electrophysical experiments needs no particularising. 
Micrometry.* — Mr. E. G. Love passes in review the various methods 
in use for determining the actual size of microscopic objects. 
The earliest efforts in this direction, such as those of Leeuwenhoek 
and Jurin, consisted in comparing the microscopic object with other 
objects, such as grains of sand, of which the size was known. 
In 1742, Benjamin Martin described an eye-piece micrometer con- 
sisting of a screw pointed at one end, and carrying at the other a hand 
which passed over a dial divided into twenty parts. 
In 1747, Cuff devised a micrometer consisting of a lattice of fine 
wire in a circular frame to be placed in the focus of the eye- piece. 
Baker made a similar micrometer of human hair. 
The needle micrometer shown in fig. 49 was designed by Adams. 
Fig. 49. 
It was clamped to the body-tube, the needle passing through a small 
opening in the eye-piece. The number of revolutions of the screw 
was registered by the prism a, while each division of the divided circle 
indicated 1/1000 in. The value of the revolutions of the screw was 
determined by means of a sectoral scale (fig. 49, A, B, and C) upon the 
stage. 
In 1840, Jackson devised an improved form of Martin’s eye-piece 
micrometer. In this micrometer the scale was on glass, and was mounted 
1896 
Journ. New York Micr. Soc., xi. (1895) pp. 97-105 . 
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