378 
MESSRS. W. E. WILSON- AND P. L. GRAY 
radio-micrometer; between the two a wooden box, 4 inches square in section, was 
placed to prevent side radiation from disturbing the latter; tin and cardboard screens 
were also used for the same purpose, until v/e were assured that the only heat falling 
on the instrument was that from the lamp-blacked side of the cube, passing through 
a carefully-cut rectangular aperture, made in cardboard and fixed to the end of the 
Fig. 7. 
wooden box close to the cube. A horizontal edge of the aperture was divided into ten 
equal parts, and a wooden screen, with a straight edge, could be placed so as to close the 
aperture, or to leave any desired fraction of it open. The proportionate area of aper¬ 
ture open, and therefore the proportionate amount of heat falling on the instrument, 
was then given by the reading of the scale on the horizontal edge ol the aperture. 
The following Table II. gives the results of two series of experiments. The first 
