ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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when at unequal distances from the centre of the apparatus, the two 
square nuts being always situated symmetrically at equal distances from 
the centre. 
The micrometers and other appliances for receiving the focal images 
are of three different kinds. 
The first kind is shown on the left-hand support of fig. 15. It con- 
sists of a small bevel-edged disc of glass, fixed in the end of a narrow 
tube, and provided with a reading lens or positivo eye-piece of about 
38 mm. focal length. Upon the outer surface of the glass disc is ruled 
a scale divided into fifths of millimetres. This micrometric arrangement 
is convenient for receiving a focal image, as formed at the back of a 
microscopic objective, at a point at some distance down the tube in which 
the objective is mounted. 
The second kind, shown in fig. 15, on the right-hand support, con- 
sists of a rather larger disc of glass, mounted in a metal rim, which is 
cut away at two places. Upon the front face of this disc, a micrometer 
scale in fifths of millimetres is also ruled, and one-half the disc is 
silvered over. This arrangement is convenient for use with larger 
lenses, and for service as an object, in which case it is illuminated from 
behind. When making determinations of microscopic objectives it is 
found convenient to use one micrometer of the second kind opposite 
the front of the objective, and one of the first kind at the back of the 
objective ; the pieces being mounted as shown in fig. 15. Each of the 
micrometers can be shifted laterally out of the lino of the lens under 
measurement, the arm which carries the micrometer being fastened to 
a collar furnished with a small handle, as shown in figs. 15 and 16. 
This collar is guided to move in a plane orthogonally to the optical 
axis of the apparatus by an adjustment of three abutting screws. 
These three abutting screws are not shown in fig. 16, but the heads of 
two of them are shown in profile in fig. 15 ; being situated behind the 
collar bearing, which receives the curved supporting stem of each of 
the two micrometers. These bearing-collars are provided with stops to 
enable the micrometers to be brought up accurately to their former 
position after having been thrown out laterally. 
A third arrangement, not shown in fig. 15, is used only for large 
lenses, and consists of a grooved metallic ring mounted on a curved 
arm upon the top of the travelling support. Over this grooved ring 
is stretched a piece of thin paper, which, after having a millimetre 
scale marked upon it, is rendered nearly transparent by the application 
of varnish, Two such micrometers are used — one at either end — when 
measurements are to be made upon camera lenses, and other lenses 
having a focal length of several centimetres. 
The holder, fitted upon tho central support, as shown in figs. 15 
and 16, for carrying the lens, is cut with tho Eoyal Microscopical 
Society’s standard screw, so as to receive any microscopic objective. 
For other small lenses an adapter, provided with the same screw, is 
used, and the lens is temporarily fitted into the adapter by means of a 
cork ring. For large lenses a Y-shaped appliance is substituted for 
tlie screw holder. 
In order to know the position of the lens itself with respect to its 
focal and principal points, it is necessary to ascertain the scale-reading 
