There are in the composition of this globe some sixty or seventy different elementary substances, and under suitable conditions each one of these substances can afford a perfectly characteristic spectrum. Thus the way of making the comparison with the nebula is to try the different elements one after another, until one can be discovered which pours forth a light that behaves under the prism as does the light from the nebula. Pursuing this inquiry, Dr. Huggins found that when hydrogen gas was ignited to incandescence by the passage of electricity, it emitted light which, after passage through the prisms, came to coincidence with one of the lines in the spectrum of nebula; and the hydrogen character of two of the other lines has been since demonstrated. It was thus established that hydrogen is one of the constituents of the Great Nebula in Orion. Further confirmation of this important discovery was forthcoming when the photographs of the spectrum of the Great Nebula were subsequently obtained. On these photographs lines were present which are constituted by light of such a nature as to be wholly invisible to the eye, though perceptible on the photographic plate. It is of the greatest interest to discover that these invisible rays from the nebula are also indicative of the presence of hydrogen. Thus we obtain a beautiful confirmation of the fact that the nebula is partly composed of glowing hydrogen. There are, however, some remaining lines, the character of which has not yet been ascertained. It would be a little premature to assert that there must be some substance in the Great Nebula not at present known to us on the earth. This would be, no doubt, one interpretation of the facts. We must, however, admit the possibility of another explanation. It is frequently found that the lines yielded by an incandescent material vary to some extent when the physical conditions of temperature and of pressure are modified. It is, therefore, not impossible that the unknown lines in the spectrum of the Great Nebula may be due to some element known to us, but which has not yet been tested under the conditions which would make it yield the particular rays we are speaking of.