132 REPORT — 1859. 



do so. With what result, the Association will have an opportunity of judging 

 by the examples exhibited*. 



Professor Phillips, aided by Mr. Bates, obtained some lunar photographs 

 in July 1853, and communicated the results of his experience in a valuable 

 paper at the Hull meeting of the Associatibnf. Mr Hartnup of Liverpool, 

 aided by Mr. J. A. Forrest, Mr. Mclnnes, Mr. Crooke, and other photo- 

 graphers, took some good pictures of the moon in 1854- J ; Father Secchi, at 

 Rome, and more recently Mr. Fry, in Mr. Howell's observatory at Brighton, 

 and Mr. Huggins, near London, have also produced lunar pictures : these 

 experiments were in all cases made with refracting telescopes, corrected for 

 the visual ray. Professor Bond, in April 1857, applied the process with 

 promise of a fruitful future, in measuring the distance and angle of position 

 of double stars §, and also in the determination of their magnitudes ; just pre- 

 vious to his decease, this new application of the art appears to have engaged 

 his attention more than lunar photography. He succeeded in obtaining pic- 

 tures of fixed stars down to the 6-7th magnitude. 



The Photographic Picture compared with the Optical Image. 



It will render what I shall hereafter have to say more easily understood 

 if I commence by bringing under notice what happens in applying photo- 

 graphy to sidereal astronomy. The optical image of a fixed star, it. will be 

 remembered, is not a mathematical but an optical point, which, in conse- 

 quence of the properties of light, is seen with the telescope as a very minute 

 disc, surrounded by rings, which become fainter and wider apart as they 

 enlarge, these rings being always more or less broken up, according to the state 

 of the atmosphere. The photographic image must, therefore, be of a certain 

 size, but it is after all a mere speck, difficult to find among other specks 

 which are seen in the most perfect collodion film, when it is viewed with a 

 magnifying power. 



For example, let it be supposed that a telescope of sufficient aperture is 

 turned upon a Lyrae ; a star conveniently situated from its great meridional 

 altitude for photography, and moreover sufficiently brilliant to give a nearly 

 instantaneous picture : if the telescope be steadily supported at rest, the 

 star will, in consequence of the earth's rotation, course along the field of the 

 telescope, in a line parallel to the earth's equator, and, as it produces an 

 instantaneous picture, the image obtained is a streak, representing the path 

 of the star. We might be led to expect, a priori, that this line, for a short 

 distance, would appear straight; but, so far from this being the case.it is broken 

 up and distorted, and consists of a great number of undulating points, 

 crowded in some places, and scattered in others. This distortion arises 

 from the disturbances in our atmosphere which cause the star to flicker. 



In the foregoing remarks, the telescope was supposed to be at rest ; now 



* The photographs exhibited at the Aberdeen Meeting were the following : — Two original 

 negatives which would bear considerable magnifying power ; two positive enlarged copies of 

 other negatives, eight inches in diameter, which would bear still further enlargement with 

 a lens of low power ; twelve enlarged positives of the Moon in different phases, 3^ inches 

 in diameter, among which were three, showing the progress of the lunar eclipse on February 

 27, 1858; enlarged positive copies of Jupiter, exhibiting his belts and satellites; lastly, a 

 photograph of Saturn and the Moon taken together at the recent occultation of that planet 

 just after the planet had emerged from the moon's bright limb (May 8, 1859). The last- 

 named photograph was produced in 15 seconds ; — a remarkably rapid result for so faint an object 

 as Saturn. The planet on this occasion was seen to be of about the same brilliancy as the 

 Mare Crisium situated near the moon's western limb, with which the planet could be readily, 

 compared, from its proximity to that lunar district. 



t Report of Brit. Assoc. 1853, Trans. Sect. A, p. 14. 



t Report of Brit. Assoc. 1S54, Trans. Sect. B, p. 66. 



§ Astronomische Nachrichten, No. 1105. 



