Some sciences are of yesterday; others stretch far back into the youth of time. Among these there is one of the beginnings of which we have lost all trace, so coeval was it with the commencement of man’s history; and that science is the one of which we have to trace the instrumental developments. Although our chief task is to enlarge upon the modern, it will not be well, indeed it is impossible, to neglect the old, because, if for no other reason, the welding of old and new has been so perfect, the conquest of the unknown so gradual. The best course therefore will be to distribute the different fields of thought and work into something like marked divisions, and to commence by dividing the whole time during which man has been observing the heavens into two periods, which we will call the Pre-telescopic and the Telescopic Ages. The work of the Pre-telescopic age of course includes all the early observations made by the unaided eye, while that of the Telescopic age includes those of vastly different kinds, which that instrument had rendered possible; so that it divides itself naturally into some three or four sub-ages of extreme importance. It is unnecessary to say one word here on the importance of the invention of the telescope; it is well for the present purpose, however, to emphasize the further distinctions we obtain when we consider the various additions made from time to time to the telescope. The Telescope, in fact, was comparatively little used until astronomy annexed that important branch of physics to its aid which gave us a Clock—a means of dividing time in the most accurate manner. In quite recent times the addition of the Camera to the Telescope marks an important advance; indeed the importance of photography is not yet recognised in the way it should be. Then, again, there is the addition of the Spectroscope, which, though it is only now beginning to yield us rich fruit, really dates from the beginning of the present century. This is an ally to the telescope of such power that he would be a bold man who would venture to set bounds to the conquests their combined forces will make. Now not only is it essential for the proper understanding of the instruments used nowadays in every observatory, by every stargazer, to go back to the origin of the science of observation, but in no other way can one fully see in what way the new instrumental methods have added themselves to the old ones. Further, it is of importance to go back to the actual old field of work in which the geometric conceptions which grew up in the minds of the men of ancient time—conceptions which we are now utilizing and extending—were gradually elaborated. To do this, there is no better way than to dwell very briefly on the work actually done by the old astronomers.