According to Dr. Hastings' view, it must be light from the sun which has undergone refraction, i.e., which has been bent from its regular course by the interposition of an opaque body like the moon.
In order to make this perfectly plain, suppose the front of a surface of waves of any sort to be striking an object which resists them. If an organ of sense is placed in the resisting object, it will judge the direction of the waves or the direction of the object producing them by a line at right angles with the wave front. Now suppose a body is placed between the body producing the waves and the sensitive organ. The waves must go around this body and will produce an eddy behind it, so that the wave front will have a different direction, and the organ of sense will conceive the origin of the waves to lie in a direction different from that before the body was interposed. Now consider the waves to be waves of light, and their origin the sun. The organ of sense is the retina of the eye. The moon is the opaque body interposed in the course of the waves, and they, being bent, make the impression on the eye that the light comes from beyond the edge of the sun. The moon covers the sun during the eclipse and a little more, so that it can move for about five minutes and still cover the sun entirely. This movement is very slight, and if the corona consists of light from a solar atmosphere, it should not change at all during this movement of the moon.
But if diffraction is the cause of the light, then the slightest change in the relative positions of the sun and the moon should change the configuration of the corona, i.e., the corona should not remain exactly the same during a total eclipse. The character of the light as shown by a spectrum analysis should change.
To determine this point Dr. Hastings invented the following instrument: Two lozenge-shaped prisms of glass were fastened in the form of a letter V, and so arranged that all the light falling within the aperture of the V was lost, and that falling on the ends of the glass prisms was transmitted by a series of reflections to the apex of the V, where the prisms touched; here was placed a refracting prism, so that the light could be analyzed. This instrument was attached to the eye piece of the telescope, and the image of the eclipse reduced to such a size that the moon just fitted into the aperture of the V, while opposite sides of the corona were reflected through the prisms to the place where they came together. In this way both sides of the corona were seen through the eye-piece at the same time. On looking at the eclipse this is what Dr. Hastings saw: The light of the corona was divided into its constituents. Prominent among them was a bright green line, which is designated by the number 1,474; to this line attention was directed. Its presence in the spectrum has been an argument in favor of the view that the corona is a solar atmosphere. If this is the case, the line should remain fixed during the eclipse; but if the corona is due to diffraction, this line should change.
It should grow shorter in the light from one side of the corona, and longer on the other. The observation was now reduced to watching for a change in the relative length of two green lines.
At the beginning of totality the line from the west side was much the longer, but as the eclipse progressed it shortened notably, while the line from the east side, shorter by about one-third at the beginning of the eclipse, grew longer. When the eclipse ended, the proportions of the lines were exactly reversed. There had been a change equal to two-thirds the length of the lines, while the sun and moon had only changed their relative positions by an extremely small amount. The only way in which this phenomenon can be accounted for is on the diffraction theory. The material view of the corona will not answer for it. But there are other discrepancies in the older view which have been known for some time. The principal ones are: 1. It is known from study of the sun that the gaseous pressure at the surface must be less than an inch of mercury, and is probably less than one-tenth of an inch, but an atmosphere extending to the supposed limits would cause an enormous pressure at the sun's surface, especially since the force of gravity on the sun is very much greater than on the earth. 2. The laws of gravitation would require a solar atmosphere to be distributed symmetrically around the sun, while the corona is enormously irregular in form.
The sun is irregular in outline, which would make its diffracted phenomena show the observed irregularity, but it is symmetrical as regards density. 3. The most interesting discrepancy of the theory of the solar atmosphere is the fact that while it is supposed to extend for millions of miles from the sun, the recent comet passed within two hundred thousand miles of the sun, and yet its orbit was not affected in the least, as it would have been if it had plowed its way through a material substance. In taking photographs of the corona it is seen to be larger as the time of exposure is longer. This shows that the corona extends indefinitely, and it decreases in brilliancy in exact accordance with the mathematical laws of diffraction. These laws involve very complicated mathematics, but by them alone Dr. Hastings has proved that there must be diffraction where the corona is, and that it must follow the same laws as those observed. There is a small envelope around the sun, but in the opinion of Dr. Hastings it does not extend beyond what is known as the chromosphere.
The question seems to be settled, with considerable certainty, that nothing exists inside of Mercury large enough to be dignified by the name of planet. There may be, and there probably are, for the perturbations of Mercury indicate it, multitudes of small masses circulating around the sun like the planets, being fragments of comets or condensations of primitive matter, whose combined luster is seen in the zodiacal light.
The other results of the work of the Commission, so far as now known, are connected with the structure of the corona, the solar appendage which extends out for millions of miles from the sun's disk. In the photographs of the Egyptian eclipse of last summer these streamers can be traced back of each other where they cross; no better proof of their extreme tenuity could be given.
The duration of an eclipse of the sun depends on three things, the distance of the sun from the earth, the distance of the moon from the earth, and the distance of the station from the equator. All of these were favorable to a long eclipse in the case of the recent one, and the six minutes of totality gave opportunities for deliberate work not often enjoyed.