If the ring of white surface were replaced by a ring of green surface, the tint being as nearly that of 1474 Kirchhoff as possible, the chance of seeing the coronal ring in that tint would be somewhat increased; and, still further, perhaps, if the field of view were examined through green glass of the same tint. It seems just possible that if prisms of triple height were used, through which the rays were carried three times, by an obvious modification of the usual arrangement for altering the level of the rays, thus giving a power of eighteen flint glass prisms of sixty degrees each, evidence, though slight perhaps, might be obtained of the presence of the substance which produces the green line. Thus variations in the condition of the corona might be recognised, and any law affecting such variations might be detected. I must confess, however, that a consideration of the optical relations involved in the problem leads me to regard the attempt to recognise any traces of the corona when the sun is not eclipsed as almost hopeless. It is clear that until some method for thus observing the corona has been devised, future eclipse observations will acquire a new interest from the light which they may throw on the coronal variations, and their possible association in some way, not as yet detected, with the sun-spot period. Even when a method has been devised for observing the gaseous corona, the corona whose light comes either directly or by reflection from solid or liquid matter will still remain undiscernible save only during total eclipses of the sun. Many years must doubtless pass, then, before the relation of the corona to the prominences and the sun-spots shall be fully recognised. But there can be no question that the solution of this problem will be well worth waiting for, even though it should not lead up (as it most probably will) to the solution of the mystery of the periodic changes which affect the surface of the sun. FOOTNOTES: [Footnote 1: The actual condition of the sun in 1842 may be inferred from the following table, showing the number of spots observed in 1837 the preceding year of maximum disturbance, in 1842, and in 1844 the following year of minimum disturbance; the observer was Schwabe of Dessau: Days of Days without New groups observation spots observed 1837 168 0 333 1842 307 64 68 1844 321 111 52 Only it should be noticed that nearly all the spots seen in the year 1844 belonged to the next period, the time of actual minimum occurring early in 1844.] [Footnote 2: The following table shows the position occupied by the years 1851 and 1860 in this report, as compared with the year 1848 (maximum next preceding 1851), 1856 (minimum next following 1851) and 1867, minimum next following 1860:--