And now that we come to the stars, whose spectra occupy minutes in taking, what we just considered will help us to understand how we can advantageously thus pass from a thousandth of a second or less, to one thousand seconds or even more, and how we can even,--given time enough,--conceivably, be able to photograph what the eye _cannot see at all_. [Illustration: FIG. 93.--A FLASH OF LIGHTNING. (FROM A PHOTOGRAPH BY DR. H. G. PIFFARD.)] We have on page 231 a photograph quite recently taken at Cambridge from a group of stars (the Pleiades) passing by the telescope. Every star is caught as it goes, and presented, not in its ordinary appearance to the eye, but by its spectrum. There is a general resemblance in these spectra from the same cluster; while in other cases the spectra are of all types and kinds, the essential distinction between individuals alike to the eve, being more strikingly shown, as stars apparently far away from one another are seen to have a common nature, and stars looking close together (but which may be merely in line, and really far apart) have often no resemblance; and so the whole procession passes through the field of view, each individual leaving its own description. This self-description will be better seen in the remarkable photographs of the spectra of Vega and Aldebaran, which are reproduced on page 235 from the originals by a process independent of the graver. They were obtained on the night of November 9, 1886, at Cambridge, as a part of the work pursued by Professor Pickering, with means which have been given from fitting hands, thus to form a memorial of the late Dr. Henry Draper. We are obliged to the source indicated, then, for the ability to show the reader here the latest, and as yet inedited, results in this direction; and they are such as fully to justify the remark made above, that minutes, by this new process, take the place of years of work by the most skilful astronomer’s eye and hand. The spectrum of Vega (Alpha Lyræ) is marked only by a few strong lines, due chiefly to hydrogen, because these are all there are to be seen in a star of its class. Aldebaran (the bright star in Taurus), on the contrary, here announces itself as belonging to the family of our own sun, a probably later type, and distinguished by solar-like lines in its spectrum, which may be counted in the original photograph to the number of over two hundred. There is necessarily some loss in the printed reproduction; but is it not a wonderful thing, to be able to look up, as the reader may do, to Aldebaran in the sky, and then down upon the page before us, knowing that that remote, trembling speck of light has by one of the latest developments of the New Astronomy been made, without the intervention of the graver’s hand, to write its own autograph record on the page before him?