again exposed it for the same length of time. After this they lowered the plate with 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 photographic 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 photographic 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 exceedingly 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 measurements, the investigator of the future will be enabled to make in his study astronomical discoveries that have hitherto escaped direct observation 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 heavens 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-
