The stars, as revealed to us by the spectroscope are all of the same order as the Sun, but they are not all of the same species. Quite a large number of stars, of which Arcturus is one of the best-known examples, show spectra that are essentially the same as that of the Sun, but there are other stars of which the spectra bear little or no semblance to it. Nevertheless, it remains true that, on the whole, stellar spectra bear witness to the presence of just the same elements as we recognize in the Sun, though not always in the same proportions or in the same conditions--hydrogen, calcium, sodium, magnesium, iron, titanium, and many more are recognized in nearly all. It is true that not all the known terrestrial elements have yet been identified in either Sun or stars; but, in general, those missing are either "negative" elements like the halogens, or elements of great atomic weight like mercury and platinum. That elements of one class should, as a rule, reveal their presence in Sun and stars wherever these are placed, and, correspondingly, that other classes should as generally fail to show themselves, indicate that such absence is more likely to be due to the general structure of the stellar photospheres and reversing layers than to any irregularity in the distribution of matter in the universe. It is easy, for example, to conceive that the heavy metals may lie somewhat deeper down within the Sun or star than those of low atomic weight. In the case of the Sun, there seems a clear connection between atomic weight and the distinctness with which the element is recognized in the spectrum of the photosphere, the lower atomic weights showing themselves more conspicuously.