ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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focus either by turning the main screw, or by the fine-motion screw on 
the travelling support. This first adjustment having been made, the 
clamps are then unfastened, and the micrometer is thrown out laterally. 
A parallel beam is then thrown in a similar fashion in the reverse 
direction through the lens, and the second micrometer is thrown into 
line and is brought by a similar process to the second principal focus. 
The plumb-line is then applied to ascertain the position of some 
fixed point of the lens, and to read oft’ the positions of the micrometers, 
which are replaced in their axial positions : or, if the zero readings of 
these are known, their positions are ascertained from the readings of 
their respective verniers. 
These three readings having been made, both the travelling supports 
are clamped to their respective nuts on the main screw. The screw 
is turned so as to cause the micrometers to travel outwards. The 
observer, looking into the eye-piece of the micrometer on the left, sees 
an inverted image of the other micrometer come into view, and, as the 
screw is turned, the micrometers reach a certain position when both 
sets of dividing lines are in focus in the same field without any parallax. 
This position can be very accurately ascertained by shifting the eye 
slightly from side to side of the lens. The two micrometers now 
occupy the two symmetric points, and their positions are observed 
either by plumb-line, or by the readings of their respective verniers. 
The simple method of calculating by subtraction of scale readings 
the true focal length / and tho distance between the two Gauss points, 
has been given above. 
Results of Measurements made with the Focometer . — The following 
examples are given of measurements made on lenses. In all cases 
here recorded a red light was used, a ruby glass being interposed. 
(1) Small Hemispherical Lens, 12 mm. aperture, by Cooke and 
Sons. — Front of hemispherical face taken as point of reference A. 
Preliminary experiments with plumb-line showed that F 2 is 76' 1 
mm. behind zero of vernier Z 2 , and F t is 49 ‘6 mm. in front of zero 
vernier Z x . 
Readings of verniers for principal foci are : — 
Z 1 = 356-55 Z 2 = 199-20 
-76-10 + 49-60 
F, = 280-45 F 2 = 248-60 
Readings of verniers for symmetric points are : — 
Z\ = 371-52 Z' a = 184-17 
-76-10 + 49-60 
S x = 295-42 S 2 
/ = mean of F 2 — S 2 & S x — F x 
233-77 
15-03 + 14-97 
" 2 
/ = 15-0 mm.; jf = 66-6 dioptries. 
* = 2(F 1 -F 2 )-(S 1 -S 2 ) 
= 2-05 mm. 
H 2 is at 263 • 6 ; at 265 • 65 of scale. 
A is plumbed at 262*65. Hence first principal point is 0-95 mm. 
within the front of the hemispherical surface. 
