helium but smashed helium atoms. It was one of the great laboratory achievements of recent times when Professor A. Fowler in 1912 succeeded in battering helium atoms in a vacuum tube sufficiently to give this kind of light, already well known in the stars. Two other rings are due to hydrogen. With these three exceptions none of the rings have yet been imitated in the laboratory. For instance, we do not know what elements are producing the two brightest rings on the extreme right and left respectively.

We are sometimes asked whether any new elements show themselves in the stars which are not present or are not yet discovered on the earth. We can give fairly confidently the answer No. That, however, is not because everything seen in the stars has been identified with known terrestrial elements. The answer is in fact given not by the astronomer but by the physicist. The latter has been able to make out the orderly scheme of the elements; and it transpires that there are no gaps left for fresh elements until we come to elements of very high atomic weight, which would not be likely to rise into the atmosphere of a star and show themselves in astronomical observation. Every element carries a number, starting with hydrogen which is No. 1, and going up to uranium which is No. 92. And what is more, the element carries its number-plate so conspicuously that a physicist is able to read it. He can, for instance, see that iron is No. 26 without having to count up how many known elements precede it. The elements have been called over by their numbers, and up to No. 84 they have all answered 'Present'.[Note: Nos. 43, 61, 75 are recent discoveries and may require confirmation. There now remain only two gaps ( 85 and 87 ) apart from possible elements beyond uranium.]

The element helium (No. 2) was first discovered by Lockyer in the sun, and not until many years later was it found on the earth. Astrophysicists are not likely to repeat this achievement; they cannot discover new elements if there aren't any. The unknown source of the two rings close together on the right of the photograph (a bright ring and a fainter ring) has been called nebulium. But nebulium is not a new element. It is some quite familiar element which we cannot identify because it has lost several of its electrons. An atom which has lost an electron is like a friend who has shaved off his moustache; his old acquaintances do not recognize him. We shall recognize nebulium some day. The theoretical physicists are at work trying to find laws which will determine exactly the kind of light given off by atoms in various stages of mutilation -- so that it will be purely a matter of calculation to infer the atom from the light it emits. The experimental physicists are at work trying more and more powerful means of battering atoms, so that one day a terrestrial atom will be stimulated to give nebulium light. It is a great race; and I do not know which side to back. The astronomer cannot do much to help the solution of the problem he has set. I believe that if he would measure with the greatest care the ratio of intensity of the two nebulium lines he would give the physicists a useful hint. He also provides another clue -- though it is difficult to make anything of it -- namely, the different sizes of the rings in the photograph, showing a difference in the distribution of the emitting atoms. Evidently nebulium has a fondness for the outer parts of the nebula and helium for the centre; but it is not clear what inference should be drawn from this difference in their habits.

The atoms of different elements, and atoms of the same element in different states of ionization, all have distinctive sets of lines which are shown when the light is examined through a spectroscope. Under certain conditions (as in the nebulae) these appear as bright lines; but more often they are imprinted as dark lines on a continuous background. In either case the lines enable us to identify the element, unless they happen to belong to an atom in a state of which we have had no terrestrial experience . The rash prophecy that knowledge of the composition of the heavenly bodies must be for ever beyond our reach has long been disproved; and the familiar elements, hydrogen, carbon, calcium, titanium, iron, and many others, can be recognized in the most distant parts of the universe. The thrill of this early discovery has now passed. But meanwhile stellar spectroscopy has greatly extended its scope; it is no longer chemical analysis, but physical analysis. When we meet an old acquaintance there is first the stage of recognition; the next question is 'How are you? 'After recognizing the stellar atom we put this question, and the atom answers, 'Quite sound 'or 'Badly smashed', as the case may be. Its answer conveys information as to its environment -- the severity of the treatment to which it is being subjected -- and hence leads to a knowledge of the conditions of temperature and pressure in the object observed.