48 Froceediiifjs of the Royal Society of Victoria. 



Photographs of the planets and stars have hitherto been 

 •obtained unsatisfactorily, and with considei'able difhculty, on 

 account of their small amount of light compared with the 

 sun or the moon. With the sun the light is so intense, that 

 the difficulty is to obtain an exposure sufficiently sliort to 

 avoid destroying the sensitive surface, and with the moon 

 even a second or two is enough for telesco])es of moderate 

 dimensions. The apparent motion of these bodies in that 

 space of time, is also so small as to require no ver}^ special 

 ■contrivances to compensate for it. With stars and ])lanets, 

 however, where the light is but an insignificant fraction of 

 even that of the moon, the time of exposure has to be so 

 much prolonged, that the earth's diurnal motion renders good 

 photographs quite impossible without tlie most delicate 

 mechanical means for keeping the telescope pointed precisely 

 and without the least deviation, on the star or planet for 

 many minutes, or even hours. For this reason, although 

 many efforts and experiments have been made in this 

 direction, it is only comparatively recently that the great 

 difficulties presented have been so far overcome as to bring 

 this department of astronomical photography within the 

 realms of practical work. The first important ste]3 towards 

 this end, was the invention of the gelatine bromide plate, 

 with its wondeifully sensitive film, reducing many times the 

 period of exposure required for the old collodion plates ; and 

 secondly, the devising of driving clocks for equatorial 

 telescopes, with automatic controlling appliances, so 

 accurately constructed that the telescope follows the motion 

 of a stai- so precisely, that a plate exposed on a group of 

 stars for an hour, will show each star as a distinct and round 

 black spot, of a size proportional to the star's brightness, 

 instead of a hlach line, which would result if the motion of 

 the telescope did not exactly correspond witli the motion of 

 the earth; and fainter stars, quite invisible to the naked eye, 

 either in the skj^ or on the i^late, are seen under the 

 microscope as minute and absolutely round black spots, 

 showing unmistakably the accuracy of the movement of the 

 telescope. These two improvements have made it possible 

 to extend the use of photography to one of the most 

 important branches of astronomy, that of cataloguing and 

 charting the stars. 



Immediately after the introduction of the gelatine films 

 in 1883, we tried to get some star photographs witli our 

 great telescope, with only partial success, owing chiefly, J 



