VII. PHOTOGRAPHY. 421 



VII. CELESTIAL PHOTOGRAPHY. 

 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. 



It was constructed, in 1857, by Ross, on the design and under the super- 

 intendence of W. De La Eue, 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 accomplished 

 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 Greenwich, where it is now superseded by an instrument of more recent con- 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 eyepiece 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. De La Hue's Model of Tower for the proposed 

 employment of Huygens' Long Focus Object-Glasses in 

 Solar Photography. Royal Society. 



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

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

 telescopic camera equatorially mounted, and 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 (composed of two achromatised meniscus lenses turned concavity 

 to concavity with an intervening space), 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 



