It remains to be considered how science may hope to obtain more trustworthy evidence than we yet have respecting the corona and its changes of form, extent, lustre, and physical constitution. In the case of the prominences, we have the means of making systematic observations on every fine, clear day. It has been, indeed, through observations thus effected by the spectroscopic method that an association has been recognised between the number, size, and brilliancy of the prominences on the one hand, and the number, size, and activity of the sun-spots on the other. But in the case of the corona, we are as yet unable to make any observations except at the time of total solar eclipse. It seems almost impossible to hope that any means can be devised for seeing the corona at any other time. Of course, without the aid of the spectroscope the corona, as ordinarily seen during total eclipses, must be entirely invisible when the sun is shining in full splendour. No one acquainted with even the merest elements of optics could hope to see the corona with an ordinary telescope at such a time. The spectroscope, again, would not help in the slightest degree to show such a corona as was shining in July, 1878. For the power of the spectroscope to show objects which under ordinary conditions are invisible, depends on the separation of rays of certain tints from the rays of all the colours of the rainbow, which make up solar light; and as the corona in July, 1878, shone with all the colours of the rainbow, and not with certain special tints, the power of the spectroscope would be thrown away on a corona of that kind. All that we can ever hope to do is to discern the gaseous corona when, as in 1871, it is well developed, by spectroscopic appliances more effective for that purpose than any which have hitherto been adopted; for all which have as yet been adopted have failed.