NATURAL PHILOSOPHY. 141 



While on the subject of lunar photography, Mr. De la Rue begged to direct 

 the attention of the Society to one or two points of physical interest, which 

 may be thus stated : points on the lunar surface having optically equal inten- 

 sity of light, do not produce equally brilliant positive and equally obscure 

 negative impressions ; the actinic rays are evidently not always in propor- 

 tion to the illuminating rays. Another curious fact, which he thought he 

 had well made out, is, that those portions of the lunar surface which are 

 illuminated by a very oblique ray from the sun, do not produce an equal 

 effect on the sensitive plate, though they are equally bright to the eye. Such 

 a phenomenon obtains during the afternoon in terrestrial photography, when 

 the sun's rays reach us obliquely through the atmosphere. The moon has 

 no visible atmosphere; nevertheless, from whatever cause it may arise, that 

 portion of the moon which is illuminated by an oblique ray, does not pro- 

 duce a corresponding effect on the sensitive plate which it does to the eye. 



By paying particular attention to the state of the collodion film, he had 

 been very successful in reducing the time of exposure, and had produced 

 pictures, not only of the lunar surface, but also of Jupiter, in from three to 

 seven seconds. The photographs of Jupiter show his belts remarkably well. 

 The beauty of the photographs exhibited of the moon, he thought it would 

 be admitted, gave great promise that at a future period photography will be 

 considered as the only correct means of mapping down the lunar surface. 

 When we shall be able to obtain collodion finer in grain and still more sen- 

 sitive, it will supersede hand-drawing altogether; and even now the results 

 obtained are much more accurate than anything hitherto done by mapping 

 or hand-drawing. It is nearly impossible, by micrometrical measurement, 

 to lay down all the details of the moon, and much, after a sort of triangula- 

 tion, has to be filled up by the eye. The work is too laborious ; and the 

 famous map of Beer and Madlcr, wonderfully accurate as it is, does not 

 fulfil the conditions of absolute accuracy in all the minute points of detail. 



The light proceeding from Jupiter possessed considerably more actinic 

 power than that of the moon, in proportion to its luminosity ; or, in other 

 words, although the light of the moon is at least twice as bright as that of 

 Jupiter, its actinic power would appear to be not greater than as from 6 to 

 5, or 6 to 4. It is not improbable that the blue tint of Jupiter may have 

 something to do with its photogenic power. It may also be stated, that the 

 darkest parts of Jupiter's surface came fully out by an exposure which did 

 not suffice to bring out those portions of the moon situated near the dark 

 limb, and consequently illuminated by a very oblique ray. 



Mr. De la Rue also stated that he had compared the pBHogenic power of 

 Jupiter and Saturn, and that, on a recent occasion, he had turned the teles- 

 cope alternately on each of these two planets, and found that to produce 

 pictures of equal intensity, the sensitized plate had, on the average, to be 

 exposed five seconds to Jupiter and sixty seconds to Saturn. Hence the 

 chemical rays from Jupiter are twelve times more energetic than those from 

 Saturn an effect undoubtedly, in a great measure, attributable to the 

 greater brilliancy of the former planet. 



OX THE ACTION OF LIGHT UPOX OXALATE OF THE PEROXIDE OF 



IRON. 



Professor J. W. Draper, in a paper on the measurement of the chemical 

 action of light, communicated to the London Philosophical Magazine, 



