The “Radiation Integrator in Vacuo” is an instrument designed by a biologist, to assist in the study of solar radiation, as received on the surface of the earth. The principle of the instrument is that a black bulb in vacuo is exposed to the sun's rays; the bulb, which contains alcohol, is connected to a graduated stem maintained at shade temperature; radiant heat from the sun causes alcohol to distil over the bulb into the stem where its volume is measured. In Samoa the shade temperature is practically constant throughout the year, but one believes on theoretical grounds that more radiation is received from the sun between September and March than at the other season, and that the radiation has two maxima, in October and February. This instrument, which has been observed for 12 months, confirms the expectation. The daily mean distillate, the distillate per hour of sunshine (Campbell Stokes) and the mean distillate for the three hours before noon, all show the same seasonal changes.

The instrument has been standardized against Gorczynski's pyrheliometer, so that the readings in c.c. of alcohol can be converted into calories. The instrument is not difficult to make or read, and it can be left in the open in all weathers. It integrates its results and requires to be read once a day in Samoa.