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SUMMARY OF CURRENT RESEARCHES RELATING TO 
corresponding to some fixed point on the lens, or on its attached mount- 
ing. This might be done by end-measurement, by bringing one of tho 
micrometers up into actual contact with one pole of the lens ; but such 
a mode of proceeding is inadvisable for several reasons. It was there- 
fore decided, when the instrument was being designed, to adopt such 
a construction as would permit of direct determinations by means of a 
delicate plumb-line. With this object, the general form of double 
girder was adopted, so that the scale might be engraved on the front 
of the vertical face. By reference to the end elevation in fig. 16, it 
will be seen that the optical axis of the apparatus is arranged to be in 
the same vertical plane. Moreover, the micrometers are so arranged 
that in each case a plumb-line can be hung directly against the actual 
face of glass or paper upon which the micrometer is engraved. As 
plumb-line, a thin silk thread with a small leaden sphere at the end of 
it is used. When the instrument is properly levelled, the plumb-line 
can be applied to read off directly on the scale the actual position of 
any of the micrometers ; and so by comparison with the readings of 
the verniers of the travelling supports is obtained the zero reading for 
use in future measurements. 
It was consequently necessary to furnish the instrument with 
levelling screws. There, however, arose a small mechanical difficulty ; 
for an instrument of this shape would not be very stable if provided with 
one foot at one end, and two near together at the other. It is therefore 
provided with four levelling screws, one at each end and two at the 
two ends of the central support. In practice, this arrangement is quite 
workable ; and it is found convenient first to adjust the level of the 
girders lengthways by the end screws, then to adjust transversely by 
the other screws. The frame is so solidly made that there is no fear 
of racking it by unequal weight upon the four screws ; the wooden top 
of a strong laboratory table is never so rigid as to make any fine adjust- 
ment necessary. 
Mode of using the Focometer . — The mode of procedure in using the 
instrument is as follows : — The lens to be measured having been 
secured in the central support, it is then adjusted in position so as to 
be accurately in the axial line between the two micrometers. The 
clamps of the two travelling supports are left loose, being only applied 
when required. One of the micrometers (in practice, that shown on the 
left in fig. 15) is then thrown out laterally, and a beam of parallel 
light is thrown through the lens (from left to right, as seen in fig. 8), 
so as to form an image at the first principal focus. In practice, this is 
done as follows : — A compound lens, which is to be used as a colli- 
mator, is placed in direct sunlight, and at the focal point, where the 
image is formed, is placed a piece of ground glass, coarsely ruled with 
black lines. When this reticle is set in its exact position with respect 
to the collimating lens, the combination is placed at one end of a long 
room, about 40 feet long, with a paraffin lamp behind the reticle to 
illuminate it. The parallel beam thus issuing from the collimating 
lens is received on the focometer about 20 feet away. 
The travelling support on the right (fig. 15) is then brought up by 
hand and adjusted so that the micrometer is approximately at the 
principal focus. It is then clamped to the screw, and, using a positive 
eye-piece to aid the vision, the micrometer is accurately adjusted to the 
