A photoelectric method of measuring the light of the night sky with studies of the course of variation through the night

The investigations already published on the intensity of the night sky have been made by means of visual photometry, using a convenient instrument with a self-contained luminous source of radioactive origin. Nothing could rival this for simplicity and portability; it is always ready and requires no attention. On the other hand visual photometry is not a very satisfactory process even for ordinary light, and with this faint light it is far from giving the desirable degree of accuracy. I have therefore spent much effort in trying to replace it by some photoelectric method of measurement. A satisfactory method has now been evolved, and will be described, together with the results. A preliminary notice of the earlier results was given in a paper written at the request of Prof. S. Chapman, F. R. S., Chairman of the International Committee on Terrestrial and Solar Relationships, the receipt of which was acknowledged by him on June 19, 1928. The relevant passage is:- "Most of the difficulties have been overcome and preliminary observations have been in progress for some months past. I have been able to follow the changes of intensity from hour to hour on clear nights. Some evidence has been found suggesting diurnal periodicity. The observed intensity nearly always increases between nightfall and midnight, beyond which the observations have not usually been carried."