228 ANNUAL OF SCIENTIFIC DISCOVERY. 



This has now been supplied bj replacing the original Munich, clock of the 

 great equatorial of the Observatory by a new one, on the principle of the 

 spring governor, invented by the Messrs. Bond. This clock, -which was 

 made by Messrs. George and Alvan Clark of East Cambridge, carries the 

 telescope with admirable evenness and regularity of motion. 



Immediately upon its completion, at the invitation of the Director of the 

 Observatory, Messrs. Whipple and Black commenced a new series of ex- 

 periments, and have succeeded in transferring to the plate, by the collodion 

 process, images of the fixed stars to the fifth magnitude, inclusive, with sin- 

 gular and unexpected precision. 



The most remarkable instances of their success are the simultaneous im- 

 pressions of the group of stars composed of Mizar of the second magnitude, 

 its companion of the fourth, and Alcor of the fifth magnitude. 



Mr. Bond then presented a series of measurements of the angular distance 

 of the companion from Mizar, taken from the plates with a micrometer 

 microscope. These measurements compared, with those given by Struve, 

 as the result of observation and calculation, showed the probable error of a 

 single photographic distance to be -j- 0". 12, or quite as small as that 

 attributed by Struve to a single direct measurement. The former method 

 has thus in its first efforts attained the limit of accuracy beyond which it is 

 not to be expected that the latter can ever be sensibly advanced. But the 

 photographic process holds out a much better promise. 



The two principal sources of error by which it is affected are spots on the 

 glass plate, or impurities in the coating in the neighborhood of the images, 

 and slight departures from symmetry in their form, as yet noticed only 

 when the plate has been exposed too long to the action of the light. The 

 latter has been the case with most of the plates from which the above 

 measurements have been taken, and they may in consequence be slightly 

 affected. It is certainly to be anticipated, that, by the exercise of more 

 care in regulating the time of exposure, the symmetry of the images can be 

 secured. A microscopic examination will in most cases serve to distinguish 

 accidental spots in the coating, or on the glass, from the molecules, which, 

 by their aggregation, show the action of light. 



The real difficulty, perhaps insurmountable, which now prevents a most 

 extensive application of photography to astronomy, is the deficient sensi- 

 tiveness of the processes in use. Unless photographs of stars as low, at 

 least, as the eighth magnitude can be obtained, its use must be restricted to 

 comparatively few double stars. Should, however, this impediment be 

 overcome, and photographic impressions be obtained from stars between 

 the sixth and eleventh magnitudes, as has already been done for those be- 

 tween the first and the fifth, the extension given to our present means of ob- 

 servation would be an advance in the science of stellar astronomy of which 

 it would scarcely be possible to exaggerate the importance. 



PHOTOGRAPHS FOR WOOD ENGRAVING. 



By a process invented by La demand, of Paris, the wood is first placed 

 with its surface on a solution of alum, and dried ; it then receives with a 



