Mars during the Late Opposition. 83 



disk of the planet, Mr. Norman Lockyer lias ascribed to the 

 presence of clouds in the planet's own atmosphere. 



Mr. Huggins in his valuable paper on the ' ' Spectrum and the 

 Colour of Mars" ( ff Monthly Notices of the Astronomical So- 

 ciety/' March 8, 1867), has referred the absence of colour at the 

 edges of the planet to some peculiar effect of the surface of 

 the planet itself. That the ochreish colour is due to some 

 peculiarity in the surface is, I think, almost proved by Mr. 

 De la Rue's exquisite drawings of Mars, as in these markings 

 of that tint are seen with definite outlines. As I have said, 

 when our atmosphere is free from mist the colour of the 

 equatoreal part is pale, and has not nearly so strongly marked 

 a ruddy tint. This is accounted for by supposing that mist 

 stops the most refrangible rays of light, that is, those towards 

 the blue end of the spectrum, whose waves have the greatest 

 velocity ; the red light thus being allowed to preponderate. 



When observed under these conditions, the edges of the 

 disk appear Naples yellow, the centre orange, tinged with 

 burnt ochre, while the parts immediately under the dark 

 markings, near the south pole, are whitish, with a tinge of 

 salmon colour. The colour of the dark markings on Mars has 

 been described as greenish or bluish grey ; they always appear 

 bluish grey to me, and with this colour I have depicted them. 

 Occasionally the north polar ice has been seen strongly tinted 

 with a bluish colour. I have examined the spectrum of the 

 planet with a direct-vision spectroscope, fitted in the eye-piece 

 of a telescope ; the spectrum of the dark markings presented 

 no distinctive peculiarity. This would scarcely prove the 

 entire absence of a blue or greenish shade of colour in the 

 markings, as I have found that if white light be reflected five 

 or six times from surfaces of metallic silver, and then received 

 on white paper, it will be strongly tinged with a chocolate 

 colour. Yet if the light reflected from white paper thus 

 illuminated be examined by means of a spectroscope, no 

 appreciable difference can be seen between the spectrum of 

 this light and that of white light. 



In consequence of the effect of irradiation, I have not been 

 able to make out satisfactorily the outline of the north polar 

 ice. I have frequently seen faint white spots appear on the 

 disk, and as these spots approached the edge of the disk, they 

 increased in brilliancy, until, when nearly at its extreme edge, 

 they almost rivalled the polar snows in whiteness ; these white 

 cloudy patches never had any definite outlines. They were 

 generally nearly circular in form, and they always appeared in 

 the region of the equator. One of these white cloudy spots 

 I have shown in the sketch taken on February 8th, at 10.30. 

 The spot is represented passing off the left hand edge of the disk. 



