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and bring it quite close to the Microscope, so that it is within arm’s 
length of the observer, and the sunbeam has so short a distance to 
pass before it reaches the substage condenser, that any slight error of 
the heliostat is of comparatively little consequence. The heliostat 
and all the accessories are fixed, once for all, on a wooden stand, so 
that they have not to be arranged each time they are used, but the 
stand has merely to be placed before the Microscope, and everything 
is in its proper relative position. 
The heliostat itself is a brass time-piece A, fig. 47, to which is added 
an additional motion, causing the spindle, which need not be in the 
centre, to revolve once in twenty -four hours. It is mounted on a 
triangular brass plate, furnished with levelling screws, and is fixed at 
an angle to the horizon, corresponding to the latitude of the place in 
which it is to be used. When the point of the brass plate is directed 
due south, and the plate itself is levelled, by means of a spirit-level, 
in both directions, the clock is in the plane of the equator, and the 
spindle, at right angles to it, is parallel to the axis of the earth, and 
points to the North Pole of the heavens. The spindle is made slightly 
conical, and fitted to it, friction-tight, so as to be capable of easy 
rotation by the hand, is a small mirror B, with universal motion. 
The size of mine is two inches by one, which is ample. This mirror 
has to be set to reflect the light from the sun in the direction of the 
spindle, when the rotation of the spindle, corresponding exactly with 
that of the earth, only in the reverse direction, compensates for the 
apparent motion of the sun, and the reflected beam remains motionless. 
Where the reflected beam crosses the optic axis of the Microscope, 
there is placed a second fixed mirror C, inclined to the horizon at 
an angle equal to half the latitude, which reflects the beam in the 
axis of the Microscope. Between this fixed mirror and the condenser 
is placed an alum-cell D, to absorb the heat. In originally fixing 
the position of the mirrors, care has to be taken that the centre of the 
fixed mirror is truly axial with respect to the substage condenser and 
Microscope, and that, reflected in it when viewed through the 
Microscope, the spindle of the heliostat appears exactly end on, in 
the centre of the field. The heliostat will then be in its correct 
position, and the movable mirror can be placed upon it. All this 
may seem very complicated in the description ; but once the position 
of the various pieces has been thus settled, ail that has to be adjusted 
is the movable mirror, and its adjustment is no more difficult than 
that of the mirror which forms the ordinary adjunct of the Microscope. 
If the mirrors are of glass silvered at the back, the first gives a 
double reflection, which is again doubled by the second, and great loss 
of light is experienced. Glass silvered on the surface avoids this, but 
I found it tarnished quickly ; so that I have had to adopt reflectors 
of speculum metal. These also are open to objection, for the light 
they reflect is distinctly reddish in tinge, and I believe there is con- 
siderable absorption of the rays of highest refrangibility. 
