I HAVE not yet seen, in any English publication, mention of the important results of the more recent researches of Herr O. Jesse and his coadjutors on these clouds. By taking simultaneous photographs from two or more widely separated places, the height of the clouds has been determined with great exactness. On July 2, 1889, this was found to be somewhat over 80 kilometres. The operations have evidently been conducted with great care, and the results may therefore be fully trusted. The question is therefore set at rest as to whether the clouds are self-luminous, for it is evident that at such a height their brightness is fully accounted for by the sun shining upon them. In 1886, Herr Jesse had, upon this supposition, ascertained their brightness to be from 49 to 54 kilometres, and that the lower the sun descended the smaller was the illumination needed to show them as the atmosphere darkened, so that the calculated height increased with the sun's depression below the horizon. Some people were incredulous about the great height at that time attributed; but the photographs give them a yet greater elevation, which places them quite out of the category of any ordinary clouds. Those who have not seen the photographs may query as to the possibility of identifying the same points in the two photographs compared, and may think that even synchronous photographs might show very different details by being taken from two distant stations; but, on the contrary, in those examples I have seen, the two photographs are so exactly alike that it is very difficult to discover any difference whatever between them, though taken at Nauen and Steglitz, 35 kilometres apart, which consideration of itself shows the enormous height of the clouds. In some of the photographs the stars α and β Aurigae are distinctly visible.