40 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Mar., 1904. 



A Photographic Atlas 

 of the Moorv. 



VoLiMi; LI. of the Annals of the Harvard College Observa- 

 tory is devoted to a photographic atlas of the Moon. This 

 marks an epoch in selenography, not only because it is the 

 first complete photographic atlas of the Moon yet pulilished, 

 but because every part of the Moon is represented under five 

 different conditions of illumination — sunrise, morning, noon, 

 evening, and sunset. This fivefold presentation is of the 

 greatest importance to the selenographical observer, as the 

 change in appearance of most of the lunar formations during 

 the course of the lunar day is so great that a photograph 

 taken at one time becomes almost unrecognisable if compared 

 with the Moon at another. It constitutes a record in another 

 particular, namely, that the entire series of photographs were 

 taken within the short period of seven months. Yet a third 

 feature of the atlas lies in that this was the first time that a 

 long focus telescope has been successfully employed in this 

 department of astronomy. Such telescopes ha\e been em- 

 ployed with success upon the corona in recent eclipses, but 

 their application to the systematic record of the Moon was a 

 new departure. 



This work is the result of one of those enterprises which the 

 measureless energy of Prof. E. C. Pickering, and the corre- 

 sponding munificence of the American public, have brought 

 to completion within the last few years. An expedition under 

 Prof. \V. H. Pickering, who has had much experience of the 

 superb observing conditions both of .-Xrequipa and Arizona, 

 was sent out from Har\ard College to the Island of Jamaica, 

 and reported so satisfactorily on the " seeing," that at the end 

 of 1900 he took out there a photographic O.G. of i^-inches 

 aperture, and 135 ft. 4 in. focal length. This was set up at 

 Mandeville, 20M0 feet abo\e the sea level, and used as a fixed 

 telescope in conjunction with a heliostat. The seeing did not 

 prove to be quite equal to expectation, and a yet more serious 

 drawback was experienced in the want of flatness of the 

 heliostat mirror. In most cases, therefore, the aperture of the 

 photographic telescope had to be diminished to 6 inches, and 

 the exposures lengthened accordingly. 



From the photographs taken by the expedition, 80 were 

 selected, each y in., by 4 in., to form the complete lunar atlas, 

 the Moon being divided for the purpose into sixteen different 

 regions, each shown, as noted above, under fi\e different con- 

 ditions of illumination. The parallels and meridians were 

 laid down on a photograph of the full Moon, taken on 1901, 

 August 21), the positions derived by Franz from measures of 

 five negatives of the full Moon take'n at the Lick Observatory 

 being taken as standards. Professor Franz's positions are 

 undoul)tedly the most accurate yet published, and Professor 

 Pickering devotes the last chapter of his book to an inquiry 

 as to whether the altitude of a lunar mountain can be deduced 

 from the discordances between its apparent co-ordinates on 

 the lunar surface, as measured under different conditions of 

 libration. The result, howe\er, is not very encouraging, the 

 displacement to be measured being very nearly of the same 

 order as the errors of observation, as Mr. S. A. Saunder has 

 recently pointed out," and a far more extensive series of 

 me.isures than any yet published are required in order to 

 satisfactorily solve the question of lunar altitudes. The third 

 chapter is devoted to the consideration of lunar change ; the 

 cases of Eratosthenes, Pliniiis, and Pallas being lightly alhided 

 to, whilst Linne, Plato, and Messier, with its companion 

 Messier A. are treated with considerable detail. The enlarge- 

 ments of Plato and Messier, especially the former, by no means 

 justify Professor Pickering's claim that " these are the first 

 photographs published, so far as I am aware, showing the details 

 of the floor so plainly th.it they may l>e clearly distinguished." 

 They are certainly not equal to the photographs of Plato 

 in MM. Loewy and Puiseux's .Atlas. Another point on which 

 Professor Pickering lays himself open to some criticism is the 

 uncompromising way in which he habitually speaks of bright 

 spots on the Moon being "snow," or "hoar frost," or " ice " 



• Observatory, iyo.|, Fobni.iry, p. y6. 



whilst dark spots are often as unhesitatingly described as 

 ■• patches of vegetation." While not wishing to ignore the 

 very considerable amount of evidence which Professor 

 Pickering has elsewhere presented in favour of these, his 

 views, they cannot yet be regarded as more than mere opinions, 

 and it is hardly legitimate for him to express himself as if they 

 were altogether beyond challenge. 



" Ann.'ils of the Astronomical Observatory of Harvard College," 

 Vol. LL, a Photogr.iphic Atlas of the Moon, by William H. Pickering, 

 Cambridge, Mass. Published by the Observatory, 1903. 



La>.rge v. Small Telescopes on 

 Planets. 



To THK Editors ok " Knowledge." 



Sirs. — I was much interested in Mr. A. Stanley Williams's 

 letter in the current number of " Knowledge." No doubt 

 there is reason in his suggestion. But to my mind there is a 

 much stronger reason for the greater relative defining power 

 of small telescopes when used on planets over their perform- 

 ance on double stars, which seems to be generally overlooked. 



( )n double stars, I Iielieve. the rule of 4-56 seconds of arc 

 divided by the aperture is generally accepted as the limit of 

 the telescope's di\'iding power, and this agrees very well with 

 theory. But it only holds good when the objects to be sepa- 

 rated are sufiiciently bright to cause strong interference 

 effects. Now the details on a planet are seen against a back- 

 ground nearly as bright, and except at the edges the contrast 

 is ^•ery feeble, so interference phenomena .are less appreciable. 

 Therefore I hold that the 4-56 seconds-over-aperture rule does 

 not apply. Mr. .\. Stanley Williams, in his first paragraph, 

 also seems to imply a doubt of ordinary rules holding for lari^c 

 areas, but I maintain that small telescopes will separate details 

 on a planet very much closer than the above rule would 

 allow. And so would, and sometimes does, a comparatively 

 large aperture, but the magnification needed to tone down the 

 light to utilize the larger aperture needs better atmospheric 

 conditions, so that it is comparatively rarely that such aper- 

 tures can be used with full effect. If we take 40 diameters to 

 the inch of aperture as about the best ratio for viewing, say. 

 Mars, one will on most nights find the seeing good enough to 

 use the 120 needed by a 3-inch. But apply that rule to the 

 40-inch Verkes, and how often can a power of 1600 be 

 employed to advantage ? 



A few years ago I made some experiments to test the sepa- 

 rating power of I inch of aperture directed to black spots on 

 white paper. I found that i inch would divide dots separated 

 not more than i second of arc. and lines 07 second apart ; 

 and that it would show a single black line o'S second in 

 width, which was, of course, separating white areas divided 

 by that amount only. I think these experiments, which 

 can readily be repeated by anyone who wishes, show 

 that when interference effects are negligible, one may expect 

 a telescope to go far beyond the usually accepted limits. But 

 if more were needed, Mr. and Mrs. Maunder have supplied it 

 in the paper published last July in the B.A.-A. Journal 

 alluded to by Mr. Williams in his letter to " Knowledge." 

 There they show that a black line on unglazcd paper was seen 

 sharply defined with the unaided eye under an angle of only 

 2'S seconds of arc. Taking the pupil of the eye when fully 

 dilated at the extreme of one quarter of an inch, this is 

 equivalent to 07 second of arc for i inch, which agrees well 

 with my own experiments detailed above, though I consider 

 it much more noteworthy, as the retina is composed of hexa- 

 gons that at the nodal point of the lens system 01 the eye 

 subtend an angle of about 23 seconds of arc, and th.at such a 

 coarse structure should show a line only 2'8 seconds wide as 

 sharply defined seems to bear out what Mr. Maunder says in 

 his last paragraph, that : " A straight line is that which gives 

 the least total excitement in order to produce an appreciable 

 impression, and therefore the smallest appreciable impression 

 oroduces the eff'ect of a straight line." 



H. W.\KE. 



Whiteha\en, January 11, 1904. 



