65 
bulbs is blacked and the other one is silvered or left clear, 
the apparatus becomes a radiometer” in the proper mean- 
ing of the term,^ that is to say, a measurer of the thermal 
radiance to which it is exposed and the intensity of which 
it indicates by variations in the angular positions of a needle 
prolonged from one or other of the radii of the wheel. 
It is only needful now to so arrange it that this needle 
shall make a tracing of its curves on a cylinder driven by 
clockwork at an even speed, and the “ radiograph” is com- 
plete. 
Concerning the actual instrument I use, its wheel is 7'3 
inches in diameter, and the weight thereof a trifle more than 
two pounds. The other portions of the apparatus are of 
the same dimensional proportions as are indicated in the 
sketch. Of course some delicate method of recording has 
to be employed, and I have thus far used the smoked paper 
process so much adopted in the observatories of France. In 
this way the radiograms” which illustrate this paper were 
obtained. 
When using the instrument to record the radiance of the 
sun I have hitherto exposed it in a box of copper sur- 
mounted by a dome of glass into which the bulbs of the 
thermometer project. The line which joins them is in the 
plane of the meridian of the place and the black bulb to the 
north. The box itself is supported at an elevation of four 
feet or thereabouts upon a stand of wood, the legs of which 
are firmly embedded in the ground. The stand itself is 
located at the extremity of a garden which overlooks a 
valley and the sea. A small window in the box permits 
the movements of the train to be seen and the promptness 
with which the apparatus acts to be observed. If a cloud 
‘‘ no bigger than a man’s hand,” and “ light as a feather” in 
* The radiometer” of Crookes should in its simple form have been 
called a “radioscope,” as it merely makes visible the elfects of radiance, 
but does not measure their amount. 
