Farrants, on a Micrometer Object-finder. 91 
exactness than is possible in reading scales applied to the 
stage which can be seen only with the eye, at most aided by 
a lens. 
I have had ample experience of the efficiency and utility 
of the method of registration, the details of which are given 
below, having used it for several years, though with very im- 
perfect scales ; indeed, they were nothing more than lines 25 
to the inch, scratched with a writing diamond on the centre 
inch of an ordinary glass slip, the vertical and horizontal 
lines being on different glasses, or on different sides of the 
same glass. These scales afforded the means of registering 
and finding any object with the §-inch object-glass, but the 
divisions were neither sufficiently regular nor minute enough 
to be used with advantage with an object-glass of higher 
power. Of late, through the kindness of Mr. G. Jackson 
(to whom I am much indebted for the trouble he has taken 
in the matter, and for which I pray him to accept my best 
thanks) , I have been enabled to make trial of different scales, 
ruled with that precision and neatness which are so well 
known to and appreciated by all who make use of the micro- 
meters ruled by that gentleman. These scales, and the mode 
of using them, I will now describe as briefly and intelligibly 
as I can. 
Since objects are usually placed as nearly as possible in 
the centre of the slide on which they are mounted, it rarely 
happens that any specimen is found within an ^th of an inch 
of either edge of the slide, and the |ths of an inch square in 
the middle of the slide is the space within which most objects 
will be found ; and since nothing is gained by having more 
lines ruled than are actually wanted, while the counting 
is thereby rendered more troublesome, it has been thought 
better not to extend the ruling over a greater surface than 
|ths of an inch square. 
The scales ruled by Mr. Jackson, which I now use, and 
would recommend, are formed by horizontal and vertical 
lines, ruled on the same surface of the glass, at a distance 
of T^o*^ vcich. apart; every fifth line is thicker, and 
consequently more conspicuous than the rest, besides, the 
centres of both series are distinguished by double lines; and 
by these means the counting is greatly facilitated : there are 
in each series 72 lines (the centres being double), which in- 
clude 70 spaces, of which 35 lie on each side of the central 
double line in each direction ; a space -/^ ('7) of an inch square 
in the middle of a glass slip is thus covered with a series of 
squares, 4900 in number, the sides of which are tJo^I^ 
inch linear, and the area to^ito^^ of a square inch ; of these 
