148 REPORT — 1859. 



time to time, photographs of the planets under the rare circumstance of a 

 quiescent state of the atmosphere. 



In photographing the planets, it is sometimes advantageous to take several 

 pictures on the same plate; this can be conveniently done with my tele- 

 scope, because the driving clock is connected with the telescope by means of 

 a peculiar spring clutch formed of two face-ratchet-wheels. When one 

 picture has been taken, the image is shut off, and the ratchet disconnected, 

 so that the telescope remains at rest, the clock continuing to go. During 

 the interval of rest, which interval is conveniently regulated by the passage 

 of a certain number of teeth of the moving half of the clutch, the planet will 

 have moved through a short distance in its diurnal arc; and when the clock has 

 been again thrown into gear, the image will fall on another part of the plate. 

 In this way, four or five images of a planet, for example Jupiter, may be 

 obtained in a very short time. These images are arranged at equal distances 

 along an arc of right ascension, and afford a ready means of determining 

 the angle of position of the belts, &c, as was proposed by the late Professor 

 Bond with respect to the angle of position of double stars. 



Relation of Actinic Poioer to Luminosity. — I have alluded before to the 

 difference in the optical and photographic picture of the moon ; another very 

 remarkable result of photography is the great difference which has been 

 proved to exist in the relation of actinic power to luminosity of the various 

 celestial objects. For example, the occultation of Jupiter by the moon, on 

 November 8th, 1856, afforded an excellent opportunity for comparing the 

 relative brightness of our satellite and that planet. On that occasion, Jupiter 

 appeared of a pale greenish tinge, not brighter than the crater Plato, and, 

 according to my estimate, of about one-third the general brilliancy of the 

 moon ; but the actinic power of Jupiter's light was subsequently found to 

 be equal to fully four-sixths or five-sixths of that of the moon*. 



Saturn required twelve times as long as Jupiter to produce a photograph 

 of equal intensity on an occasion specially favourable for making the experi- 

 ment; yet I obtained a picture of Saturn together with that of the moon in 

 15 seconds on May the 8th of the present year, just as the planet emerged 

 from behind the moon's disc. The picture of the planet, although faint, is 

 sufficiently distinct to bear enlarging. 



With two pictures of the moon and a planet (or a bright fixed star) taken 

 at a short interval at the period of an occultation, or near approach of a 

 planet or star by the moon, we may obtain a stereoscopic picture which 

 would make the moon (seen, of course, as a flat disc) appear nearer than 

 the planet or star. 



Stereoscopic Pictures of the larger Planets. — Photographs of the planet 

 Jupiter, although far inferior hitherto to the optical image seen with an eye- 

 piece, show the configuration of the belts sufficiently well to afford us the 

 means of producing stereoscopic pictures; all that is necessary is to allow 

 an interval to elapse between the taking of the two pictures, so as to profit 

 by the rotation of that planet on its axis. In the space of 26 minutes the 

 planet will have rotated through the 15° 48' necessary to produce the 

 greatest stereoscopic effect. 



Mars would, in 69 minutes, have rotated through the same angle, and, as 

 his markings are veryt distinct, we may hope to obtain stereoscopic views of 

 that planet. 



The markings on the other planets are too faint to hold out a promise of- 

 similar results. Although this is the case with respect to Saturn, the ap- 



* Monthly Notices Roy. Ast. Soc. vol. xviii. p. 55. 



