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STRONOMERS are at present endeavouring to become fully acquainted with the resources of a new tool which has recently been placed in their hands. Perhaps it would be rather more correct to say that the tool is not exactly novel in principle, but that it is rather the development of its capabilities and its application in new directions which forms the departure now creating so much interest. We have already learned much by its aid, while the expectation of further discoveries is so well founded that it is doubtful whether at any time since the invention of the telescope the prospects of the practical astronomer have seemed so bright as they are at this moment.

In the earlier periods of astronomical research it was the movements of the heavenly bodies which specially claimed attention, and it was with reference to these movements that the great classical achievements of the science have been made. But within the last two or three decades the most striking discoveries in observational astronomy have been chiefly, though by no means exclusively, concerned with the physical constitution of