2 REPORT 1872. 



series gave the direction of the dailj^ motion of the heavens. The probable 

 error of a single measurement of the photographic distance of the images was found 

 to be +0"'12, or somevphat smaller than that of a direct measurement with the 

 common filar micrometer. The late Professor Bond, who applied photography to 

 stellar astronomy, confining himself to stars brighter than the seventh magni- 

 tude, discussed the results in various numbers of the ' Astronomische Nachrichten.' 

 No astronomer more unbiased could have been selected to decide on the comparative 

 value of the photographic and direct observational method. His discussion shows that 

 the probable error of the centre of an image was +0"- 051, and that of the distance 

 of two such centres was +0"-072. Adopting the estimate of Struve, +0"-217, as the 

 probable error of a single measurement of a double star of this class with a filar 

 micrometer, Professor Bond shows that the measurement of the photographic images 

 would have a relative value three times as gi-eat. He derived the further important 

 conclusion, that deficiency of light can be more than compensated for by propor- 

 tionate increase in the time of exposure. A star of the ninth magnitude would 

 give a photographic image, after an exposure of ten minutes, with the Cambridge 

 equatorial. 



In the reproduction of stars by photogi-aphy, recently undertaken by ]Mr. Ruther- 

 furd, the objects to be secured being so minute, special precautions were found to 

 be necessary in depicting them upon the sensitive film, so that their impressions 

 might be distinguishable from accidental specks in the collodion plate. To prevent 

 any such chance of mistake, Mr. Rutherfurd secures a double image of each luminary, 

 the motion of the telescope being stopped for a short time (half a minute) between 

 a first and second exposure of the j)late ; so that each star is represented by two 

 close specks, so to speak, upon the negative, and is clearly to be distinguished by 

 this contrivance from any accidental speck in the film. A map of the heavens is thus 

 secured, very clear though delicate in its nature, but yet one upon which implicit 

 reliance can be placed for the purposes of measurement. Professor Peirce aptly 

 says, " This addition to astronomical research is unsurpassed by any step of the 

 kind that has ever been taken. The photographs afibrd just as good an opportunity 

 for new and original in'\estigation of the relative position of neai- stars as could be 

 derived from the stars themselves as seen through the most powerful telescopes. 

 They are indisputable fiicts, unbiased by personal defects of observation, and which 

 convey to all future times the actual places of the stars when the photographs 

 •were taken." 



Mr. Asaph Hall, who shared -with Professor Bond the work of measuring 

 the photographic images and of reducing the measurements, has very re- 

 centlj' suljjected the photographic method to a critical comparison, with a view 

 to deciding on its value when applied to the observation of the transit of Venus. 

 He appears, as regards its application to stellar observations, to underestimate the 

 photographic method in consequence of want of rapidity ; but he admits that in the 

 case of a solar eclipse, or of the transit of a planet over the sun's disk, it has very 

 gi'eat advantages, especially over eye-observations of contacts, inner and outer, of 

 the planet and the sun's limb, and that the errors to which it is subject are worthy 

 of the most thorough investigation. The observation of a contact is uncertain on 

 account of irradiation, and is also only momentary ; so that, if missed from any 

 cause, the record of the event is irretrievably lost at a particular station, and 

 long and costly preparations lendered futile. On the other hand, when the sky 

 is clear, a photographic image can be obtained in an instant and repeated through- 

 out the progress of the transit ; and even if all the contacts be lost, equally valuable 

 results will be secured if the data collected on the photographic plates can be cor- 

 rectly reduced, as will be proved hereafter to be undoubtedly possible. That the 

 transit of Venus will be recorded by photography may now be announced as certain, 

 as preparations are energetically progressing in England, France, Russia, and Ame- 

 ricafor obtainingphotographic records. There is also a probability of Portugal taking 

 part in these observations ; for it is contemplated by Seiior Capello to transport the 

 Lisbon photoheliograph to Macao. There are at present five photoheliographs in 

 process of construction for the observing parties to be sent out by the British 

 Government, under the direction of the Astronomer Royal, Sir George B. Airy. 

 The Russian Government will supply their own parties with three similar instru- 



