SIR. WILLIAM HUGGINS CAMPBELL. 313 



I looked into the spectroscope. No spectrum such as I expected. A single 

 bright line only. At first I suspected some displacement of the prism, and that 

 I was looking at a reflection of the illuminated slit from one of its faces. This 

 thought was scarcely more than momentary. Then- the true interpretation 

 flashed upon me. The light of the nebula was monochromatic, and so, unlike 

 any other light I had as yet subjected to prismatic examination, could not be 

 extended out to form a complete spectrum. After passing through the two 

 prisms it remained concentrated into a single bright line, having a width cor- 

 responding to the width of the slit, and occupying in the instrument a position 

 at that part of the spectrum to which its light belongs in refrangibility. A little 

 closer looking showed two other bright lines on the side toward the blue, all 

 the three lines being separated by intervals relatively dark. 



The riddle of nebulae was solved. The answer, which had come to us in the 

 light itself, read : Not an aggregation of stars, but a luminous gas. Stars after 

 the order of our own sun, and of the brighter stars, would give a different 

 spectrum ; the light of this nebula had clearly been emitted by a luminous gas. 

 With an excess of caution, at the moment I did not venture to go further than 

 to point out that we had here to do with bodies of an order quite different from 

 that of the stars. Further observations soon convinced me that, though the 

 short span of human life is far too minute relatively to cosmical events for us 

 to expect to see in succession any distinct step in so august a process, the 

 probability is, indeed, overwhelming in favor of an evolution in the past, and 

 still going on, of the heavenly hosts. A time surely existed when the matter 

 now condensed into the sun and planets filled the whole space occupied by the 

 solar system, in the condition of gas, which then appeared as a glowing nebula, 

 after the order, it may be, of some now existing in the heavens. There remained 

 no room for doubt that the nebula?, which our telescopes revealed to us, are the 

 early stages of long processions of cosmical events, which correspond broadly to 

 those required by the nebular hypothesis in one or other of its forms. 



Further observations identified two of the lines as due to hydro- 

 gen. Observations by various spectroscopists have increased the 

 number of bright lines known to exist in nebular spectra to 30 or 40, 

 but aside from hydrogen and helium, accounting for about one-half 

 of all the observed lines, the chemical constitution of the so-called 

 gaseous nebulae is unknown. 



To leave the subject of the nebular spectrum here would mislead 

 the inexperienced, and it is necessary to say that only a minority of 

 the nebulas thus far observed in this way show spectra consisting 

 chiefly of bright lines. The spiral nebulas have spectra chiefly con- 

 tinuous, and their composition and physical state remain a mystery. 

 Even so for bright-line nebulas, as observed by Huggins in 1864, we 

 cannot say that they are shining by virtue of the heat of incandes- 

 cence; the tendency of present-day opinion is that their substances 

 are comparatively cool, and that their luminosity must arise from 

 other conditions not now understood with certainty. 



Important contributions to our knowledge of temporary stars — the 

 so-called new stars — were made by Huggins in half a dozen papers 

 on their spectra. The principal stars studied were those which ap- 



