74 THE POPULAR SCIENCE MONTHLY. 



again exposed it for the same length of time. After this they lowered 

 the plate wirfa the telescope to the same extent as they had before 

 shifted its position, and then, for a third time, exposed for an hour. If, 

 after this, the original were to be examined with a microscope, it would 

 be seen that each little star is really composed of three points, which 

 form a small triangle. Thus any doubt is dispelled that might have 

 been entertained as to whether an accidental blur had been pictured. 



The advantage in preparing representations of the heavens by 

 means of photography rests not only on the fact that by this means 

 charts of the stars can be obtained much more readily than was the 

 case when each star had to be separately noted, but the pictures thus 

 obtained also seem to be absolutely correct ; they contain no faulty 

 entries, no mistakes. Even the most attentive observer is liable to 

 error ; he may overlook one or more stars, he may make a wrong 

 entry, etc. All of these risks are not to be feared in employing a pho- 

 tographic plate ; it is like a retina that sees everything as it is ! This 

 advantage can not be sufficiently appreciated, for it enables us to leave 

 to coming generations an absolutely true and entirely correct picture 

 of the starry heavens of to-day. The director of the observatory at 

 Paris has for this reason suggested the obtaining of a complete pho- 

 tographic picture of the entire heavens by the systematic co-operation 

 of different observatories in the northern and southern hemispheres. 

 This is, indeed, a grand project ; and to see it realized would, at all 

 events, require a period of from eight to ten years but what exceed- 

 ingly important results would ensue from this ! 



With such charts from different times at his disposal, and equipped 

 with a microscope and a micrometric apparatus to carry out his meas- 

 urements, the investigator of the future will be enabled to make in his 

 study astronomical discoveries that have hitherto escaped direct obser- 

 vation by the telescopes of the observatory. In his study he will be 

 able to prove whether any, and, if so, which stars have changed their 

 position in the heavens, whether among the countless number of the 

 faintest little stars in the milky-way new ones have arisen, or old 

 ones disappeared in short, with the aid of such charts there opens 

 to the mind a vista of research and discovery that seems well-nigh 

 endless. 



How much may be escaping astronomical science of to-day, simply 

 because the eye of mortal explorer chances not to alight on that very 

 point in the depths of the heavens where just then a most important 

 event is taking place ! 



In future this will be different. Photographed charts of the heav- 

 ens give an exact likeness of the appearance of the celestial dome at 

 the time of their taking, and these may be examined and studied at 

 any place and at any time, by day and by night. The most remote 

 planet that revolves around the sun, known of to-day, is Neptune ; 

 yet it seems most probable that beyond this, one or even more plan- 



