phenomenon, it yet leaves very little room for doubt as to the general characteristics of the corona.

We are fortunately able to dispose very briefly of some of the theories respecting the corona which were suggested in old times. We need no longer inquire with close scrutiny into the theory that the corona is due to a lunar atmosphere, because we now have abundant evidence that either there is no lunar atmosphere, or that at least no atmosphere competent to produce such a remarkable appearance surrounds our satellite. We know that two very definite results (to consider no others) must inevitably follow if the Moon had an atmosphere of even moderate extent. In the first place, the refractive power of such an atmosphere would cause somewhat more than one-half of the Moon's surface to be illuminated-precisely as, in the case of our own Earth, the Sun is apparently raised by atmospheric refraction above the horizon of places lying beyond the hemisphere turned directly towards him. It is easy to show that under these circumstances, when the Moon is nearly new, her horns should extend somewhat beyond a semicircle. The fact that no such extension has been noticed suffices to prove that she has either no atmosphere or one of very limited extent. Again, the occultation of a fixed star by the Moon could not fail to be accompanied by evidence of the existence of any lunar atmosphere. Instead of disappearing suddenly, the star would be slowly reduced in brilliancy, and would appear to cling for a few moments to the outline of the Moon's disc. Since no such

appearances are noted, we must reject the conception that the Moon has an atmosphere of appreciable extent, and with it the theory, which to Kepler and Halley had seemed attractive, that the corona is a phenomenon due to the action of a lunar atmosphere on the solar rays.

Nor need we dwell on the theory propounded by Delisle, that the corona may be an optical effect due to the diffraction of the solar rays as they pass by the Moon, because Professor Baden Powell and Sir David Brewster have abundantly demonstrated that the effects due to such diffraction could not be discernible from the Earth.

We may thus limit our attention to two general theories (each admitting of special differences) which at present divide attention. One is the theory that the corona is a solar appendage; the other is the theory that it is a phenomenon due to the passage of solar light though our own atmosphere.

It will be seen that somewhat important issues depend on the selection we have to make between these two theories. For, if the corona be but a phenomenon of our own atmosphere, it is not worthy of more attention than we might give to the rays which stream through openings between clouds and form vast beams of light across the heavens. But if it be a solar appendage, then it is one of the most imposing phenomena the

Halley mentions that contrary sentiments were entertained by those whose judgments he should always revere.' It has been supposed that Halley here refers to Newton.

mind of man can dwell upon. Those long beams have (then) a real extension compared with which the volume of our Earth, nay even the volume of the Sun himself, sinks into utter insignificance; and that inner radiance which encloses the Sun on every side indicates a luminous region of inconceivably vast extent, while the problems suggested for our consideration by the aspect of this region, and by the physical state of the material distributed through it, are of the most interesting character.

6

It will conduce perhaps to clearness of conception if we consider where that portion of our atmosphere is situated which, according to what may be called the atmospheric glare' theory-advocated by M. Faye, Mr. Lockyer, and possibly by others-is illuminated by solar light during total eclipses. We can then inquire at our leisure by what path the solar light reaches this region of our atmosphere.

Let the observer be at o (fig. 86) in the centre of the Moon's shadow bb', which forms an elliptical dark space on the Earth's surface. We need not concern ourselves with the shape of this ellipse, which will vary in different eclipses and at different stations. We need only note that in a considerable total eclipse the least diameter of b b' will be greater than 100 miles. Now, let a 'represent a portion of the Moon's shadowcone, forming within our atmosphere a figure not differing greatly from an oblique cylinder. Assigning to the atmosphere a height of about 200 miles this cylinder will have a shape such as a b'; but if we assign to the

atmosphere an elevation of 500 miles, then we should have to assign to our shadow cylinder the figure a b'. Now, let lines drawn from the observer's eye to the boundary of the Moon's disc enclose the black cone shown in part in o m, while lines drawn to the boundary of a circular corona extending one degree on every side of the Moon's disc during totality form the cone shown

in part in o c c. Both cones are shown well within the cylinder a b', because as a matter of fact we find that the lines o a, o a' would contain an angle considerably greater than the angle c o c'.

We are here considering, be it remembered, the atmosphere which is effective in reflecting solar light to the eye; and it will scarcely be admitted by most meteorologists that an atmosphere of this sort extends even to so great a height as 100 miles from the Earth's surface. Our best estimate (so far as this characteristic of the atmosphere is concerned) is undoubtedly that founded on the height of the twilight curve when observed from suitable stations, for this height depends on the very quality we are considering. Now Bravais, from a discussion of Lambert's observations of the crepuscular curve, deduced a height falling short of 100 miles, while his own observations made from the summit of the Faulhorn gave a height of about 66 miles. So far as the real extension

Now, if the atmospheric glare theory is true, all the cone o c c' in our atmosphere is illuminated at the time. of central eclipse except only the core belonging to the cone o m. This is certain, because we see the Moon dark and the corona bright; so that we require Om to be dark and the remainder of o c c' to be bright. Now, so far as undeflected solar rays are concerned, the whole region a b' is in shadow. The light from the prominences can get into this region, and so perhaps can solar light deflected by some possible action at the Moon's surface. But the problem which the supporters of the atmospheric glare' theory have to solve is to get the light into the cone o c c', growing brighter and brighter up to the very boundary of the dark cone mo (to correspond with the increase of the corona's light up to the Moon's limb), and there suddenly ceasing. This done, they must show further that if another observer is stationed somewhere else

of the atmosphere is concerned, we may accept the opinion of Dr. Balfour Stewart that observations made on the aurora supply the best means of forming an opinion. From such observations made in 1819. Dalton estimated the extreme height of the auroral arch at 102 miles. Sir John Herschel estimated the height of the auroral arch seen on March 9, 1861, at 83 miles. Observations of meteors afford another means of solving the problem. A height greater than any of those above mentioned has been deduced from observations of this sort. Lastly, polariscopic observations have led Liais and others to the conclusion that our atmosphere extends to a height of more than 200 miles from the Earth. The rarity of the atmosphere at such an elevation assuming the law of diminution of density which prevails lower down to continue unchanged, would be altogether inconceivable. A quantity of air which a healthy person of average height could draw into his lungs at a single inspiration would suffice, when so reduced in density, to fill a sphere exceeding in diameter the orbit of Jupiter.