Mars

Apart from the slight modifications on which these observations turn, no alteration in the Fraunhofer spectrum of the sun has been observed in the light reflected from Mars. There is however a very marked change in the distribution of light through the spectrum. All observers agree that the more refrangible part of the spectrum is relatively weak, and to this fact is due the planet's ruddy colour. Exceptions are to be noted in the cases of the polar caps, and dark areas, for the spectrum of the caps continues in great strength into the ultra-violet, and the bluish green colour of the dark areas is due to the predominance of light of short wave length. In view of the generally negative spectroscopic revelations of an atmosphere, and the ease with which features of the planet's surface, more especially the polar caps, are seen in the telescope, it has until quite recently been generally believed that the at mosphere of Mars is extremely tenuous. Evidence not altogether corroborative of this view is supplied by photographs taken by light of different colours.

Photographs by Light of Different Colours.

The study of Mars by the method just referred to involves a kind of analysis that has had some use in general scientific observation, and which may be said to occupy a place between precise spectroscopy, on the one hand, and the more versatile means of analysis provided through the perception of both form and colour by the eye, on the other. A series of photographs taken with approximately homogeneous light of distinct spectral regions has an advantage over the coloured presentation provided by ordinary visual per ception in that its several images are separate, but its testimony, like ocular evidence, must be interpreted in the light of independ ently derived knowledge. Moreover, it is a matter of common knowledge that any object of diversified colour will suffer an apparent alteration in the contrasts of its several parts if the colour of the light by which it is viewed is changed. This is a natural consequence of the phenomenon of colour. Photography provides a specially suitable means for observing such alterations because it renders them in uniform terms of photographic density. Very marked differences are occasionally found when the colour used in photographing is altered, and it is sometimes possible, through a study of these differences, to learn something of the body under observation. Differences in the aspect of Mars when observed in this way appear first to have been described by de la B. Pluvinel and F. Baldet, and by G. Tikhoff, in 1909, but their important observations seem generally to have been over looked. The phenomena were rediscovered and more com

pletely described by later observers, especially on the planet's approach in 1924. A series of photographs, taken by light of five different regions of the spectrum, is shown in figs. 1 to 5. The spectral positions of these regions are sufficiently well defined by the names of colours attached to the illustrations, except in the ultra-violet and infra-red, where the wave-lengths are approxi mately 3700A and 7600A respectively. The photographs are arranged in the order in which they were taken, which is, except in respect to fig. 5, also the spectral sequence of the colours used. It will be noted that there are progressive changes throughout the series 1 to 4. Those which relate to the position of the markings are readily recognized as being due to the planet's rotation on its axis. The axis passes upward to the right, and approximately through the centre of the polar cap, which is seen on some of the photographs as a small white dot on the planet's edge; and the effect of rotation is to displace the markings of successive images upward to the left. After allowing for these displacements, there remain differences of a more fundamental character which will now be considered. Figures 4 and 5 were taken so closely together in point of time that they provide substantially identical presen tations of the planet, and the disparity in their appearance is one that it would be very difficult to explain as a consequence of the different colours of its several parts. Reduced contrast of mark ings against the bright background would be expected in a violet photograph because of their bluish green colour, but the total obliteration of all structure shown in the infra-red image, and the substitution of something of an entirely different kind, seems difficult to explain as a colour effect. Before attempting an inter pretation it will be well to summarize some of the more remark able peculiarities which pictures of this kind exhibit when taken, as during recent favourable approaches of the planet, with refined instruments and great precision of method.

(I) Excepting the polar caps and limb light, the familiar features of the planet are invisible in the ultra-violet photographs, but appear, and become increasingly conspicuous, as the wave length of the light used in observation increases.

(2) The polar caps and limb light are, on the other hand, strong in the ultra-violet photographs, and weaken with increasing wave length.