Page:Popular Science Monthly Volume 86.djvu/438

434 Vega: Altair$$=$$3:7, which is at variance with the results obtained on Mt. Hamilton, and emphasizes the importance of making observations through an atmosphere free from water vapor. He concluded that with a more sensitive galvanometer and one of the largest reflectors it would be possible to observe stars to the fourth magnitude.

An extensive series of measurements of the radiation from Arcturus, Vega, Jupiter and Saturn were made by Nichols by means of his radiometer, which, like the thermopile, absorbs all the radiations of all wave-lengths falling upon it. The receivers were 2 mm. in diameter. A candle at a distance of 1 m. would have given a deflection of 724 mm. He used a two-foot reflector and observed deflections of 1 to 2 mm. The sensitivity of his radiometer was such that a deflection of 1 mm. would be caused by 1/68,750,000 of the heat received on a surface equal to the aperture of the concave mirror from a candle at 1 meter distant. Or, neglecting atmospheric absorption, the sensitivity was such that by using the two-foot mirror to focus an image of the flame upon the radiometer, he would have obtained a deflection of 1 mm. from the candle placed at a distance of 5 miles. He concluded that the thermal intensity was Vega: Arcturus: Jupiter: Saturn$$=$$1:2.2:4.7:0.74. As for the possibility of further work he concluded that by using a five-foot reflector it would be possible to observe white stars down to the second magnitude and red stars possibly to the third magnitude.

The Boys radiomicrometer has also been tried in measuring radiation from stars. The instrument was used with a sixteen-inch reflecting telescope. The slight deflections obtained on various planets and stars were regarded as of questionable origin.

The earliest measurements of the light from stars by means of a selenium cell were made by Minchin, who used a 2-foot reflector. He examined about a dozen stars, some being as small as the third magnitude. Owing to the peculiar properties of the selenium cell, which is highly selective in its response to radiations of different wave-lengths, the data can not be used in comparing the radiation from different stars. The selenium cell can be applied, however, in the measurement of the maximum and minimum of light emission from a variable star which does not change in color. For this purpose it has been used by Stebbins in connection with a 12-inch refractor.

The telescope used in the present investigation was the well known Crossley Reflector which is part of the equipment of the Lick