ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 147 



Value of Photograph/ in the Production of Selenographical Charts. 



Pictures of Copernicus may be cited as an example of the aid photography 

 would afford in mapping the lunar surface : this becomes especially apparent 

 when an original negative is examined with a compound microscope. The 

 details brought out in and around this crater in a fine negative by a three-inch 

 object-glass are quite overwhelming from their number and variety. Not only 

 the elaborate network of sinuous radiating lines on the exterior of Copernicus, 

 but also the terraces in the internal walls of that wonderful volcano, the double 

 central cone, the curvature of the sole of the crater, and its polygonal form, 

 all appear in vigorous outline. 



Again, photographs of the Apennine ridge, under different illuminations, 

 are among the most beautiful of the results of the application of the art to 

 selenography ; it renders conspicuously evident many details of tint and form 

 in that extensive ridge, which would escape the most careful scrutiny of the 

 visual picture unless attention was previously directed to them by the pho- 

 tograph. Unaided by photography, it would indeed be almost hopeless to 

 attempt a correct representation of that wonderful chain of mountains, affected 

 as its form is, on account of its vast extent, by libration, and also on account 

 of the changes in the shadows occasioned by the varying direction of the 

 illumination. Aided by my collection of pictures, I hope to be able to acquit 

 myself in a creditable manner of the trust I have accepted, and to contribute 

 that quota of the lunar surface allotted to me by the British Association. 



If, at a future period, the entire lunar surface is to be again mapped down, 

 photography must play an all-important part, for, as Messrs. Beer and Madler 

 remarked in their invaluable work on the moon, it is quite impossible to com- 

 plete even a tolerably satisfactory representation of our satellite in those rare 

 and short moments when the mean libration occurs. One is therefore obliged 

 to observe the moon under many different conditions of libration, and to 

 reduce each measurement and sketch to the mean before the mapping can 

 be proceeded with ; for not only the position, but also the shape of the 

 objects is altered by libration even from one evening to another. On the 

 other hand, with photography at command, we may obtain in a few seconds 

 pictures of the moon at the epochs of mean libration, and accumulate as 

 readily a great number of records at other times. The latter would furnish, 

 after reduction to the mean, a vast number of normal positions with which 

 the more minute details to be seen with the telescope might be combined. 



By means of a microscope, with a camera-lucida prism fixed on the eye- 

 piece, enlarged drawings are readily made of different dimensions by varying 

 the magnifying power and the distance of the paper from the eye-piece ; with 

 a normal magnifying power of seventeen times linear, drawings of lunar 

 craters can be conveniently made of the exact scale used by Beer and Madler 

 for the large edition of their maps, by simply placing the drawing paper at 

 the proper distance. These drawings may then be rendered more complete 

 from time to time by filling in the minuter details by actual observation, 

 and in this way materials accumulated for a selenographical chart such as 

 even the skill and perseverance of a Madler could not hope to accomplish. 



Photography of the Planets. 



Occasionally I take photographs of the fixed stars, and among others have 

 made pictures of the double star Castor, but, as a general rule, I leave the 

 fixed stars under the able custody of the Harvard Observatory, Cambridge, 

 U.S., and devote my attention chiefly to the moon, making, however, from 



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