By its rotation the sun itself offers opportunities for bringing the stereoscope to bear upon it. Two pictures, taken at an interval of twenty-six minutes, show just the amount of difference needed to give, by their combination, the maximum effect of solidity.[448] De la Rue thus obtained, in 1861, a stereoscopic view of a sun-spot and surrounding faculæ, representing the various parts in their true mutual relations. "I have ascertained in this way," he wrote,[449] "that the faculæ occupy the highest portions of the sun's photosphere, the spots appearing like holes in the penumbræ, which appeared lower than the regions surrounding them; in one case, parts of the faculæ were discovered to be sailing over a spot apparently at some considerable height above it." Thus Wilson's inference as to the depressed nature of spots received, after the lapse of not far from a century, proof of the most simple, direct, and convincing kind. A careful application of Wilson's own geometrical test gave results only a trifle less decisive. Of 694 spots observed, 78 per cent. showed, as they traversed the disc, the expected effects of perspective;[450] and their absence in the remaining 22 per cent. might be explained by internal commotions producing irregularities of structure. The absolute depth of spot-cavities--at least of their sloping sides--was determined by Father Secchi through measurement of the "parallax of profundity"[451]--that is, of apparent displacements attendant on the sun's rotation, due to depression below the sun's surface. He found that in every case it fell short of 4,000 miles, and averaged not more than 1,321, corresponding, on the terrestrial scale, to an excavation in the earth's crust of 1-1/5 miles. Of late, however, the reality of even this moderate amount of depression has been denied. Mr. Howlett's persevering observations, extending over a third of a century, the results of which were presented to the Royal Astronomical Society in December, 1894,[452] availed to shatter the consensus of opinion which had so long been maintained on the subject of spot-structure.[453] It has become impossible any longer to hold that it is uniformly cavernous; and what seem like actually protruding umbræ are occasionally vouched for on unimpeachable authority.[454] We can only infer that the forms of sun-spots are really more various than had been supposed; that they are peculiarly subject to disturbance; and that the level of the nuclei may rise and fall during the phases of commotion, like lavas within volcanic craters.