ON THE EFFECTIVE TEMPEEATURE OF THE SUN. 
371 
visible, and by means of whicli heat can be allowed to fall upon them. If desired, 
any or all of the tubes may be stopped by means of corks. 
In an experiment, a short tube is inserted in the opening in the water-jacket 
opposite to the receiving surface, on which the heat from the platinum is to be 
allowed to fall; the mouth of the tube is partially closed by a stop of polished brass, 
in which is a circular hole, 4’94 millims. in diameter ; the size of the aperture 
was carefully measured by means of a micrometer gauge. The distance of the aper¬ 
ture from the receiving surface w'as also carefully measured, and is equal to 
60'2 millims. 
This gives for the angle subtended by a diameter of the aperture at the receiving 
surface, 4°702.* 
This number is a constant for any position of the strip, and is equal to the 
apparent diameter of the disc of glowing platinum as seen from the receiving surface ; 
the distance of the platinum strip, therefore, may be altered without affecting the 
reading of the radio-micrometer, provided that it be not so great that the angle sub-, 
tended by its width is less than that subtended by the aperture. In the hole in front 
of the receiving surface, on which the heat of the sun falls, a brass tube, 8 centims. 
long, and blackened inside, is inserted to cut off side radiation. A wooden box covers 
the entire instrument during an experiment, the box containing holes opposite 
to those in the water-jackets. By this means the instrument is completely protected 
both from draughts and from accidental radiation from lamps or other sources 
of heat in the room. 
Fig. 5 is from a photograph, showing the radio-micrometer and meldometer in 
position, with the protecting wooden cover of the former removed. 
The Heliostat. 
The heliostat used was a single-mirror instrument of Professor G. Johnstone 
Stoney’s design. The mirror was a thick piece of plate glass, with a plane surface 
carefully figured by Sir Howard Grubb. It was unsilvered, and well blacked at the 
back, and was of such dimensions that it subtended an angle at the radio-micrometer, 
when inclined at its usual angle during our experiments, only a little larger than that 
subtended by the sun. The sunlight from the mirror passed through a small hole in 
the shutter of the laboratory window, and by this arrangement the heat from the sky 
round the sun was completely cut off; thus no measurements had to be made, as in 
Professor Bosetti’s work, to obtain the effect of sky radiation. 
The use of a single-mirror heliostat w^as essential, on account of the irregularities 
produced by polarization in the intensity of the beam reflected from two surfaces, as 
well as from the difficulty of measuring the two angles of incidence in a two-mirror 
form. 
* See note on p. 391. 
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