194 PROGRESS OF ASTRONOMICAL PHOTOGRAPHY. 



member Mr. Hartniip. In 1855 the Rev. J. B. Reade, who has dis- 

 tinguished himself by his discoveries in photography, obtained special 

 notice and honorable mention at the Paris exhibition for his photo- 

 graph of the moon. Others also have been taken at Rome by Signer 

 Padre Secchi, at Brighton by Mr. Fry, and in the vicinity of London 

 by Mr. Huggins. All these photographs possess merits of their own, 

 and give decided promise of future and greater success. 



Admiral Smyth, in the Speculum HartivelUaniim, pp. 249, 250, and 

 285, speaks of Mr. Bond's labor in celestial photography, particularly 

 pointing out that in 1857 a photograph was sent to the astronomer 

 royal taking in the whole field between Mizar and Alcor, with such 

 exactitude as to show their angles of positions and distances. 



Mr. Do La Rue's success in obtaining photographic pictures of the 

 moon possessing great sharpness of definition and accuracy of detail 

 is owing to the happy combination of a variety of causes. Possessing 

 a large mirror of such exquisite defining power that but few existing 

 telescopes equal it in accuracy of definition, and brought into figure 

 by his oivn hands, and by peculiar machinery of his oiun contrivance, 

 he was at once freed from those imperfections in the actinic image 

 which are of necessity inherent in the very best refractors, even when 

 corrected most accurately for chromatic dispersion. 



Mr. De La Rue at first had no clock-work apparatus to govern the 

 motion of his telescope, and, after making several successful lunar 

 photographs with the aid of the hand-gear of the telescope, he dis- 

 continued his selenographical experiments until he had removed from 

 Canonbury to Cranford — a change of residence which, for the in- 

 terests of astronomy, he had for some time previously in contempla- 

 tion. He then furnished his telescope — his own in a double sense — 

 with a clock-work apparatus, which from time to time has passed 

 through numerous alterations,' and which is still in course of improve- 

 ment. The mechanical problem before him, as the fellows of this 

 society well know, was one of extreme complexity; for not only must 

 the motion of the clock-work be perfectly smooth and equable, but it 

 must also be capable of acceleration and retardation, to keep pace, so 

 to speak, witli the ever-varying velocity of the moon in the heavens — 

 a variation compounded of its diurnal motion and its ever-changing 

 velocity in its orbit. 



Lastly, by a rare and happy combination of chemical with mechani- 

 cal skill, the time necessary for the exposure of the collodion film 

 was materially shortened. The final result is this, that images of the 

 moon have been repeatedly taken in the focus of the mirror, admit- 

 ting of very considerable amplification, and exhibiting details on the 

 moon's surface sufiiciently clear to admit of delineation under a mi- 

 croscope provided with a camera lucida, and thereby furnishing ma- 

 terials for a more accurate selenography than has heretofore existed. 

 Neither must we altogether omit that by stereoscopicall}^ combining 

 images of the moon, taken in different phases of her librations, more 

 particularly enlarged copies, eight inches in diameter, Mr. De La Rue 

 has brought to light details of dykes, and terraces, and furrows, and 

 undulations on the lunar surface, of which no certain knowledge had ! 



