(3) Ultra-violet and violet photographs record at times whitish areas and mottlings which have been found to be temporary. These, like the polar caps and limb light, decrease in strength with increasing wave-length.
(4) When the planet is near opposition there is a pronounced fading, or darkening, at the limb in the red and infra-red images. The violet and ultra-violet images are, on the contrary, usually brightest at the limb.
(5) The ultra-violet and violet images are noticeably larger than those taken by red and infra-red light.
The most systematic observations of Mars by this method were undertaken during the oppositions which occurred during the years 1924 and 1926.
The evidence has therefore been available for only a short time, and it is doubtful whether anything approximating a consensus of opinion in relation to its interpretation exists. In lieu of a more authoritative statement an explanation which has already been tentatively advanced by the author will be offered.
The theory may conveniently be approached through a consider ation of figs. 9 and 10, which show small sections of a distant landscape photographed by light of the colours used in 4 and 5 directly above them. Obliteration of the distant part of the land scape in 10 is known to be due to the earth's atmosphere, and the comparison of the two pairs of photographs is suggestive of the presence of an atmosphere of considerable optical density on Mars. The light in the upper half of 1 o comes from the atmos phere intervening between the camera and the distant snow-capped mountains. This part of the picture therefore is, in a sense, an image of the atmosphere, and if the analogy with the Martian pictures is correct the violet photograph, fig. 5, is an image of the Martian atmospheric shell. The infra-red photograph, fig. 4, is quite obviously one of the planet's surface. Under this inter pretation the excess in size of the violet and ultra-violet images referred to in (5) p. 956, is explained, because a shell must be larger than the body which it contains. Half of the difference between the diameters of the two images should obviously be the height of the Martian atmosphere, or rather the height to which it can be photographed, and this height has been provision ally estimated from the measures of the diameters as being of the order of 6o miles, though a discussion of all available material may greatly alter this estimate; indeed, a number of writers are averse to accepting so high an estimate of the atmospheric thick ness (see Menzel, Astrophysical .1 own., vol. 63, p. 58, 1926;
Fessenkoff, Astrom. Nach., vol. 228, p. 25, 1926). The principal source of uncertainty in such a determination is encountered in the measurement of the infra-red image, which at the favourable time of opposition presents a very weak edge, especially if a dark marking falls on a part of it which may be selected for meas urement. An implication of the foregoing theory, that has been seriously disputed, relates to the polar-caps. These features are larger and stronger in the violet and ultra-violet photographs than in those taken by the infra-red light, and if the former are, in fact, pictures of the atmosphere, it seems difficult to escape the conclu sion that the caps are in part, atmospheric phenomena. This is a conclusion that a number of excellent observers of the planet are unwilling to accept, for the reason that they regard it as incom patible with the telescopic appearance of the caps, and with their observed behaviour.
A word must be said about Martian clouds. These have been known for a great many years, but so much light has been shed upon them by the recent photographs that they may properly be discussed under this head. There is substantial evidence leading to the belief that Mars has clouds of two kinds. What we shall call the first kind (see figs. 1 and 5) are specially conspicuous on photographs taken by light of the violet end of the spectrum, and are not seen on the red photographs ; those of the second kind (see fig. 7 at the "stag's throat") have an exactly contrary be haviour, being strong in the red photographs and invisible in the violet. The term cloud is here used in a very general sense and is meant to designate merely an area of atmospheric unhomogeneity which registers a cloudlike impression on a photograph, though there is reason to believe that clouds of the second class may actually be aqueous clouds. The writer has suggested that clouds of the first class lie high in the Martian atmosphere, and those of the second at a lower level.