ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 149 



parent opening and closing of his ring as lie revolves round his orbit affords us 

 the means of obtaining a stereoscopic picture. Thus photographic reductions 

 of the two original drawings which 1 made in November 1852 and March 

 1856 placed in the stereoscope (in such a manner that the major axes of the 

 rings'are at right angles to the line joining the eyes) give a picture in which 

 the planet appears as a spheroid encircled by his system of rings, although 

 the difference of position of the two pictures amounts only to 7°. And there 

 is no reason why we may not obtain a stereoscopic picture composed of 

 photographs taken actually from the planet. 



Loss of the Actinic Rays by Reflection. 



Until very lately, my celestial photographs were obtained by placing the 

 sensitized plate at the side of the tube, opposite to the diagonal reflector of 

 the Newtonian telescope ; hence the light, before it reached the plate, was 

 twice reflected. As it requires a very firm support for the diagonal specu- 

 lum, of even a 13-inch mirror, to prevent vibration, the arm carrying this 

 mirror was firmly screwed to the side of the telescope-tube, and rendered im- 

 moveable ; I could not therefore make experiments intakingthe pictures direct, 

 that is to say, with the light only once reflected, without some alteration to 

 the diagonal holder. I have, however, within the last few months, contrived 

 an apparatus which permits of the ready removal and replacement of the 

 diagonal mirror without impairing its stability, and celestial pictures are now 

 taken at will, either direct or reflected out at the side of the tube ; more- 

 over it requires but a minute to change the apparatus to produce either 

 result. With these means, I am able to make experiments to determine the 

 relative actinic intensity of the light after one or two reflections. The ex- 

 periments are still in progress, and have been begun so recently, that it is 

 scarcely advisable to hazard a conjecture as to the result ; but I may say 

 that I am disappointed as to the increased rapidity of the production of 

 a celestial picture by the direct method over the twice-reflection method ; 

 and I am inclined to infer that Steinheil's result as to the loss by reflection 

 from speculum metal of the luminous ray does not hold as regards the 

 actinic ray. 



In concluding the first part of this report, I would remark that to photo- 

 graph the moon continuously is a laborious undertaking, and affords full 

 occupation for one observer, who must not fail to pay unremitting attention 

 to the condition of the various chemicals employed, so as to be always pre- 

 pared for a fine night with such as will work. 1 would therefore strongly 

 urge the claims of this new branch of astronomical science to a more ex- 

 tended cultivation than it has hitherto received, with the conviction that it 

 will require the ardent co-operation of many astronomers to develope lully 

 its rich resources. 



Part II. — Photoheliography at the Kew Observatory. 



The Photoheliograph erected at the suggestion of Sir John Herschel* 

 at the Kew Observatory has already been described in the Reports of the 

 Kew Committee, 1856-57t and 1858t. and in the Report for the present 

 year. 



It will not, however, be out of place to give some account of the instru- 

 ment as at present actually in use, for, whilst part of the apparatus originally 



* Report Brit. Assoc. 1851, p. xxxiv. t Id. 1857, p. xxxiv. * Id. 1858, p. xxxiv, 



