ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 139 



to alter the going of the telescope without changing the rate of the pendulum. 

 This I propose to do by substituting an arrangement, similar to that known 

 in mechanism as the disc and plate, for the wheel-work now connecting the 

 machinery of the clock with the pendulum ; the disc and plate being capable 

 of producing a variable motion, according as the disc is nearer to or farther 

 from the centre of the plate. The pendulum will, by the proposed plan, be 

 driven by frictional contact, and, having employed this system in other 

 machinery, I feel persuaded that its application to the clock-driver will not 

 be attended with difficulty or inconvenience. 



The moon, besides her motion in right ascension, has also a motion in 

 declination, which is greatest when she is situated in one of the nodes formed 

 by the intersection of the plane of the moon's orbit and the plane of the 

 earth's equator, and is least when situated 90° from these nodes, where it 

 vanishes. As this motion is at times very considerable, it is evident that, 

 with a telescope made only to rotate round the polar axis, the best results 

 will be obtained, all other circumstances being alike, when the motion in 

 declination is at zero. Assuming that, on the average, 15 seconds are 

 necessary for taking a lunar photograph, the moon may have shifted upwards of 

 4" of arc in declination during that period ; and evidently many details would 

 be lost and the others considerably distorted. In order to ensure the most 

 perfect results under all circumstances, it is desirable to give a movement to 

 the declination axis of the telescope simultaneously with the movement of 

 the polar axis. Hitherto, so far as I am aware, no means have been devised 

 to effect this, but the requisite adjustable motion might be transmitted by 

 means of the disc and plate above described, from the driving-clock, although 

 its pendulum moves with a uniform velocity. 



Lord Rosses Method. — In my original method of taking the pictures by 

 means of the sliding eye-piece before spoken of, both motions in right 

 ascension and declination were provided for by adjusting the slide in the 

 diagonal parallel with the moon's apparent path. Lord Rosse, at a subse- 

 quent period, applied a clock-movement to such a slide, and made some 

 experiments in celestial photography*; but, the telescope being required for 

 other special purposes, it appears that they were not long continued. This 

 motion of the plate-holder does not meet all the exigencies of the case, but 

 if one of his magnificent reflectors were arranged to move bodily along a 

 guide adjustable in the direction of the moon's path, by means of some such 

 mechanism as I have alluded to, I believe that lunar pictures might be 

 produced of exquisite beauty, because defects in the collodion film and the 

 glass plate would be of less consequence than with telescopes of shorter 

 focal length, the image being larger in the ratio of focal length ; for example, 

 even with the three-foot instrument it would be 3 inches in diameter. 



Degree of Perfection hitherto attained in Lunar Photography. 



In my own telescope, the picture of the moon is only about l-pjin. in dia- 

 meter ; it might be suggested that the image could be enlarged by means of 

 a combination of lenses before reaching the sensitized plate, but this would 

 have the effect of prolonging the time of exposure, and moreover introduce 

 the disadvantages of the refracting telescope, and the result would not be so 

 good, for even if the moon's motion in declination were followed automa- 

 tically, still the outstanding atmospheric disturbances before alluded to would 

 remainf. Indeed, if the aperture of the telescope could be considerably 

 increased in relation to its focal length, much finer pictures would be 

 procured, because the time of exposure would be shortened. In practice it 



* Monthly Notices of the Roy. Ast. Soc. vol. xiv. p. 199. 

 t Ibid. vol. xviii. p. 1 7. 



