THE LIGHT. 33 
mination from the clear blue sky is no longer 
sufficient, and it is necessary to use a heliostat. 
This instrument carries a mirror, by which the 
direct rays of the sun are reflected in a line which 
corresponds with the axis of the achromatic con- 
denser. A clock-work apparatus causes the mir- 
ror to follow the sun in such a manner that, when 
properly adjusted, the reflected light continues to 
fall in the same direction, notwithstanding the 
constantly changing position of the source of 
illumination. It is essential when working with a 
heliostat that the apparatus for making photo- 
micrographs be arranged in a window facing the 
south, for this instrument must be placed upon 
the meridian, with its mirror facing to the north. 
When adjusted for latitude, placed on the meridian 
in a perfectly level position, and set to the true 
time, the heliostat throws the light continuously 
upon one spot, if the clock is regulated with pre- 
cision. But in practice, this is rarely accomplished, 
and a heliostat regulator is required to centre the 
light just before making an exposure ; for if the 
light is not exactly centred, the field is unequally 
illuminated. 
Not only the actinic rays of the sun, but the 
heat rays as well are reflected by the heliostat ; 
and being brought to a focus by the achromatic 
condenser, they would quickly destroy the object 
and melt the balsam mountings of the lenses of 
the objective, if some way were not provided to 
arrest them. The problem is to arrest or throw 
aside the heat rays without interfering with the 
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