

VII. PHOTOGRAPHY. 



VII. CELESTIAL PHOTOGRAPL 

 a. INSTRUMENTS. 



1852. The Kew Photo-heliograph, or Telescope, em- 

 ployed at the Kew Observatory for taking photographs of the sun's 

 disc. Kew Committee of the Royal Society, Kew Observatory. 



A telescope, constructed for the purpose of obtaining photographs of the 

 solar disc. It was constructed, in 1857, by Ross, on the design and under the 

 superintendence of W. De La Hue, Esq., at the cost of the Royal Society ; 

 and erected at the Kew Observatory, where occasional sun pictures were 

 taken by its means until 1860, when it was dismounted, and taken to Spain, for 

 the purpose of photographing the solar eclipse of that year. This it accom- 

 plished most satisfactorily, and a full account of its work was published in 

 the Philosophical Transactions. 



On its return to England, Mr. De La Rue established it at Cranford, where 

 during the year 1861, almost daily, solar photographs were taken with it. 



In 1862 it was again removed to Kew, and there maintained in constant 

 operation until 1872. In 1873 it was transferred to the Royal Observatory, 

 Greenwich, where it is now superseded by an instrument of more recent con- 

 struction. 



The diameter of the object glass is 3^ ins., and its focal length 50 ins. 

 An Huygenian eye-piece is employed for magnifying the image, and the 

 instantaneous exposure of the plate is effected by causing a sliding plate, 

 containing an aperture variable at will, to be rapidly drawn across the focus 

 by a strong spring, which is released from the top by cutting a thread. 



1852a. Photo-Heliograph, constructed by Dallmeyer, and 

 used for taking Photographs of the Sun. This' consists of a 

 telescopic camera equatorially mounted, driven by clockwork. 



The Astronomer Royal. 



The telescopic camera, total length about 8 feet, is made of brass tubing 

 5 inches in diameter, parallel for a length of 6 feet, when it opens out into a 

 cone of about 2 feet in length, and sufficiently large at its extremity to receive 

 the camera-screen, or sensitized plate, 6 inches square. 



The object glass, of 4 inches aperture and 60 inches focus, corrected for 

 coincidence of chemical and visual foci, occupies the other end of the tube 

 furnished with the means of adjustment for focussing. The sun's image, 

 produced by the object-glass at its focus, measures about half an inch in 

 diameter, when it is enlarged, by a system of lenses termed a secondary 

 magnifier, to 4 inches on the camera screen. The secondary magnifier has 

 all the necessary appliances for adjustment of focus. The difficulty to be 

 surmounted in this arrangement is " optical distortion " in the enlarged 

 image, which is, happily, almost entirely overcome. 



Coincident in position with the small sun's image, formed by the object- 

 glass, are perforations in the tube for the admission of sliders, containing 

 apertures with cross-wires, glass reticules, &c. respectively ; each capable 

 of being placed concentric with the small image. At the same place also is 

 the instantaneous shutter arrangement for effecting the exposures. This 

 consists of a metal slide, perforated by a slit-opening. The shutter is actuated 

 at one end by a spring, while at the other end a string, passing over a pulley 

 and attached to a hook, can be made to hold the spring in a state of tension. 

 This done, on the thread being cut or burnt, the spring is alloAved to act, the 

 shutter flashes across the image forming a cone of rays, exposes the sensitized 

 plate, and the picture is produced. There are provisions for regulating the 



