Our Astronomical Column

Abstract

MONOCHROMATIC PHOTOGRAPHS OF PLANETS.—Prof. R. W. Wood has given an account of further results obtained by the photography of celestial bodies through filters transmitting limited regions of the spectrum (Astrophysical Journal, vol xliii., p. 310). After much preliminary work, successful photographs were readily obtained when the 6o-in. reflector at Mount Wilson was placed at his disposal for four nights during last October. For the ultra-violet filter a bromine cell was used, transmitting the region from 3500 to the end of the solar spectrum at 2900; the infra-red screen transmitted the region above 7000, the yellow screen all rays above 5000, and the violet from 4000 to 4500. In the case of Saturn the pictures taken through the infra-red screen only showed the merest traces of the belts ordinarily seen, while through the yellow screen the planet presented its usual appearance. On the plates taken with violet light a very broad, dark belt surrounded the planet's equator, and a dark cap of considerable size was shown about the pole. These features were also present in ultra-violet light, but were less pronounced; they may possibly be due to the existence in the planet's atmosphere of some substance capable of absorbing violet and ultra-violet light. Another point of interest was a decrease in contrast between the inner and outer ring as the wavelength of the effective light decreased, suggesting that the outer ring contains so much finely divided matter that it shines in part by diffusion. In the case of Jupiter, the dark belts were scarcely visible on the infra-red plates, while they were shown in greatest contrast with violet light. The dark polar, cap shaded off gradually in the yellow and infra-red pictures, but was sharply terminated in the violet and ultra-violet photographs. It is hoped that it may be possible to investigate the surface of Mars by this method at the next near approach to the earth.