1864.] 



President's Address, 



511 



as well as the markings on the surfaces of some of the principal planets. 

 It has also been employed to measure the apparent distances and 

 angles of position of double stars, and to record the time of the sun's 

 passage over the wires of a transit instrument with much greater 

 accuracy than could be done by an actual observer ; and it is probable 

 that the number of the useful applications of the art to astronomy 

 has by no means reached its limit. 



Among those who have successfully cultivated this comparatively 

 new department of astronomy, it may perhaps be fairly said that Mr. 

 De la Eue stands preeminent. It is not that he claims any priority in 

 making this application of the photographic art. Several astronomers 

 have made experiments in celestial photography, and some of these 

 earlier than Mr. De la Eue, but no one, I believe, has devoted himself 

 so systematically and assiduously to overcome the many difficulties 

 w^hich are met with in the process, and no one has been so successful 

 in the results which he has obtained, particularly in regard to the sun 

 and moon, his photographic pictures of which bodies are sufficiently 

 delicate in their details to admit of the most precise measurement. No 

 one who has not seen Mr. De la Eue's pictures of the moon can form 

 an idea of their exquisite sharpness and beauty of definition. No doubt 

 part of the superiority of these pictures is due to his employment of a 

 reflecting telescope of exquisite defining power, the large mirror of 

 which was figured by his own hands, and by peculiar machinery of his 

 own contrivance. Thus he entirely avoided those imperfections of 

 the actinic image which arise from outstanding chromatic dispersion in 

 the very best refractors, especially when, as usual, they are achroma- 

 tized for the luminous and not for the chemical rays of the spectrum. 



The late Professor Bond, of Cambridge, in the United States, with 

 the assistance of Messrs. "Whipple and Black, of Boston, was the first 

 to make a photographic picture of any heavenly body. In the year 

 1845 he obtained good pictures of a Lyrse and of Castor, and in 1850, 

 by means of the great reflector of the Harvard Observatory, he obtained 

 a daguerreotype of the moon, which was placed in the Exhibition of 1851. 

 It was the sight of this which first gave the impulse to Mr. De la 

 Eue's labours in this direction. 



At the latter end of 1852, Mr. De la Eue took some successful 

 positive lunar photographs on collodion by means of an equatorially 

 mounted reflecting telescope of 13 inches aperture, made by him- 

 self; and he w\^s probably the first to employ in celestial photo- 

 graphy the then recently discovered collodion process, which is that 

 now exclusively adopted. At that period he had not applied any 

 driving motion to the telescope, and was therefore obliged to follow 

 the apparent motion of the moon by means of a sliding frame fixed in 

 the eyepiece holder, and moveable by the hand ; but in 1857 he a]3plied 

 a driving clock to the telescope, the rate of which could be adapted to 



