June 1, 1896.] 



KNOWLEDGE 



133 



An examination of ice-crystals with a good lens will 

 show their immense variety. Replying to the question 

 " why are some halos concentric with the sun, while 

 others pass through the disc ■? " Kaemptz says: " If it were 

 possible to study the crystals of snow that reflect and 

 refract the light, it would be easy to reply to that question. 

 It appears to me probable tl;at these diflerences are due to 

 different forms that the crystals may assume. If they are 

 prisms floating in the air, a luminous circle is produced 

 by refraction ; if they are lamellated crystals, the 

 reflection produces a parhelic circle passing through the 

 sun." Now, those who, like the present writer, have 

 carefully examined a shower of ice-crystals, will know 

 the endless varieties that fall in the same shower. It 

 is not, therefore, surprising to find both kinds of halos at 

 times represented in one phenomenon. Such appearances 

 are recorded in treatises on meteorology, and an instance 

 occurred, as recorded by Mr. C. Wood, of Middlesborough, 

 on the evening of -Tuly 22nd, 1894, at Eedcar. The 

 present writer witnessed a very similar appearance on 

 •Tuly 11th, at Westnewton, Aspatria. Mr. Wood speaks 

 of circles of " cirrus form of cloud." I need hardly 

 remind the reader that these geometrical figures are due 

 to optical law, and were no cloud visible the result would 

 be the same, and the colours more perfect, if the crystals 

 were there in sufficient quantity. This I have confirmed 

 by a variety of observations. 



Speaking of the quantity of these ice-crystals floating in 

 the air, a remarkable observation of the sun pillar is given 

 by Lohrmann, seen near Dresden, on the evening of one 

 day and the morning of another, and repeated on several 

 occasions afterwards. The calculation was made some 

 time ago that if all the vapour contained in solution by the 

 atmosphere fell in snow, the whole earth would be covered 

 to a depth of twenty feet. This gives us an idea of the 

 countless multitudes of crystals which may be present in 

 one perspective view of the heavens. In accounting for 

 the sun pillar in its vertical form, I have cited, as 

 analogous, the band of light reflected from still water. It 

 may be objected that the particles of water are continuous, 

 but the atmospheric ice-crystals are not so. But the 

 particles of water are not ahsohitchj continuous — indeed, 

 no particles of any form of matter are — and the number of 

 ice-prisms is quite sufficient, at times, to produce the 

 optical effect now described. A more closely related 

 instance of an apparently continuous band reflected from 

 separated points is to be found in the beam of motes 

 projected into a darkened room through the aperture of 

 a shutter. 



In conclusion, we may note that these optical phenomena 

 of the white rainbow and the sun column derive additional 

 interest and significance from the continuance of the 

 " historic " frost with which they have recently been 

 associated. 



. THE PROGRESS OF SELENOGRAPHY. 



By Arthur Mee, F.R.A.S. 



ANY moonlight night two hundred and fifty years 

 ago, whilst the good people of Uantzic were 

 wrapped in slumber, one of the wealthiest 

 burghers of the Prussian city might have been 

 found deeply engaged in the contemplation of 

 our satellite. His observatory was very difl'erent from 

 what we associate with the name, and his clumsy and 

 imperfect telescope a modern amateur would cast aside 

 ■with contempt. But though his apparatus— the most 



perfect of his day — was to our way of thinking absurdly 

 inadequate, Hevelius possessed those qualities which make 

 the successful observer, and his great skill, his splendid 

 energy, and his untiring perseverance made up in no small 

 degree for the imperfection of his instruments. Night 

 after night he watched, drawing after drawing was 

 carefully secured, until, at the end of several years, he 

 found himself possessed of suflicient material to lay down 

 a chart of the moon. Here, again, our astronomer spared 

 no pains in the execution of his task. He drew his map 

 eleven inches in diameter with minutest care, he engraved 

 it on copper with his own hand, he wrote a treatise on 

 what lie had discovered, and he gave the whole to the 

 world in the shape of a folio volume with a ponderous 

 Latin title, after the fashion of the times. Quaint in the 

 extreme is this old map, with its mountains in perspective, 

 its lands, its oceans, and its rivers all carefully christened 

 by the author, and last, but not least, the winged cupids in 

 the corner bearing scrolls descriptive of the chart and its 

 abbreviations. 



Such was the earliest map of the moon. Galileo had 

 observed the spots, Scheiner and Langrenus had drawn 

 them, but it was left to Hevelius to construct the first 

 chart which, though of no great value to the modern 

 student, must ever remain a monument to the patience 

 and skill of the father of selenography. It was not long 

 before Hevelius' map had a rival in that of Riccioli, of 

 Bologna, prepared from Grimaldi's observations. Hevelius 

 had been content to name the various objects from 

 presumed terrestrial analogies ; but Riccioli, with a touch 

 of human vanity, conceived the idea of christening the 

 various formations after the philosophers of his own and 

 of ancient days. In carrying out this scheme of nomen- 

 clature, the Bolognan observer was careful to include his 

 name as well as that of Hevelius, and, as Eiccioli's system 

 was accepted, the pair are still found side by side on our 

 modern maps of the moon. 



For a whole century the chart of Hevelius remained the 

 chief lunar authority. Cassiui and others had, indeed, 

 made attempts in the same direction, but no advance was 

 recorded until Tobias Mayer, of Gottingen, determined on 

 a new map. During the lunar eclipse of 1748 he felt 

 the want of an accurate chart, and set to work to prepare 

 one in twenty-five sections. He had collected his materials 

 when death cut short his work at the early age of thirty- 

 nine years. A small map embodying his observations 

 was, however, issued; but the large one did not see the 

 light till a few years ago, when it was produced under the 

 editorship of Klinkerfuss. 



Another long interval elapsed, and then Schroeter, of 

 Lilienthal, applied himself to the study of the moon. He 

 did not construct a map, but he published a series of 

 careful drawings, as he tells us, with the express object of 

 leaving to his successors materials for settling the question 

 of lunar change. Up to this time, and, indeed, until the 

 publication of the great work of Beer and Miidler, the 

 moon was considered to be a living world, with lands and 

 seas, and rivers — with forests and active volcanoes, and, 

 perhaps, with sentient bemgs inhabiting its surface. 

 Schroeter's labours, therefore, were invested with peculiar 

 interest, and were carried out with all the zeal and earnest- 

 ness that characterized those of Hevelius, but with instru- 

 ments far superior to those which the older philosopher 

 possessed. Miss Agnes Gierke justly styles Schroeter the 

 originator of selenography in its modern sense, and his 

 vicissitudes, no less than his devotion to astronomy, 

 deserve that the story of his most interesting life and 

 labours should be given in detail to the world. Schroeter 

 was not a skilful draughtsman, but he made his designs 



