October 23, 1885.] 



SCIENCE. 



363 



explanation usually given ^ of this phenomenon is 

 that it is due to the refraction of the sun's rays pass- 

 ing through their atmospheres, and thus illuminating 

 rather more than one hemisphere at a time. Any 

 small body surrounded by a ring of light would, 

 naturally appear darker by contrast than the sur- 

 rounding background. 



In regard to photographing the moon in the daytime, 

 it may be as well to call attention first to the fact, 

 that as the meon and sky are nearly of the same 

 brilliancy, and there are accordingly no irradiation 

 effects, it is not a question of the best form of appara- 

 tus, but almost entirely of the contrast qualities of 

 the plate and developer employed. In fact an ordi- 

 nary camera furnished with a long focussed land- 

 scape lens is as good an instrument as can be de- 

 vised for this investigation. Fortunately I had on 

 hand some of Edwards's bromide plates, imported last 

 June, and they, together with some Carbutt B. and 

 Anthony chloride plates, were employed in the fol- 

 lowing determinations. 



It should also be stated in regard to my remark, 

 reading " the impossibility of photographing the moon 

 in the daytime, v/hen the sun is high above the 

 horizon," that this was merely a general statement, 

 founded on observations made in June and July 

 when the sun's altitude in the middle of the day 

 was between 60" and 70°. 



Dr. Huggins has now shown that this statement is 

 not rigidly exact, as with the sun at an altitude of 

 35', and the moon in the most favorable position at 

 this season (the third quarter), he has obtained a 

 distinct image upon his plates. I repeated his ex- 

 periment, October 16 and 17, when the moon was 

 in the first quarter, and' with the sun at an altitude 

 of 18' obtained a similar result. The images, though 

 distinct, w&re far too faint to print, and only two 

 plates out of nine showed any image at all, although 

 the moon was very conspicuous to the eye. I should 

 consider it doubtful if photographs of the moon could 

 be obtained with the sun at an altitude of over 60°. 

 If then there is difficulty in obtaining an impression 

 of the moon at 90' distance from the sun, how much 

 more difficult would it be to photograph the still 

 fainter coronal rays, when masked by the dazzling 

 brilliancy of our atmosphere in the sun's immediate 

 neighborhood . 



But what particularly interested me in Dr. Hug- 

 gins's communication was, that I saw at once that 

 it furnished me a new constant, and accordingly 

 a new method, for determining the relative light of 

 the atmosphere near the sun, and the corona. Five 

 separate measurements were made between 1.15 and 

 4.15 on October 16 and 17, of the relative light 

 of the sky in the immediate vicinity of the sun and 

 moon, by the photographic method described in a 

 previous paper.- These ratios varied from 16, when 

 the sun was highest, to 50 at the later hour. Tak- 

 ing the average of these values, we may safely 

 assume that between three and four o'clock, when 

 my successful pictures of the moon were taken, the 

 light about the sun is generally not far from 35 times 

 as bright as the light of the sky in a region where it is 

 just possible to photograph the moon. But accord- 

 ing to the observation of Prof. S. P. Langley, pre- 

 viously quoted, the light of the moon is ten times 

 that of the corona at 3' distance from the sun. 

 Accordingly the light of the atmosphere in the im- 

 mediate vicinity of the sun would have to be reduced 

 1 Newcomb's Astronomy, p. 299. 2 Science, Aug. 14. 



350 times in order to obtain an impression of the 

 corona upon our plates. If the sun were at a greater 

 altitude, this figure would be somewhat smaller. The 

 value found by my previous experiments was 320. 

 The closeness of the coincidence is probably acci- 

 dental, but of the two methods the first one seems to 

 me rather the more accurate. 



Wm. H. Pickering. 



Voss-Holtz electrical machine. 



In response to Mr. Eaton's communication in 

 Science, No. 141, I would say that, about a year 

 ago, I compiled for one of my classes a discussion of 

 the Voss-Holtz electrical machine. Some months 

 afterward Mr. E. B. Benjamin prepared a pamphlet 

 regarding his machines, and asked my permission to 

 incorporate what I had given my students regarding 

 the theory of these. I granted his request, though 

 not satisfied with the completeness of the discussion. 

 What I had written had not been intended as a con- 

 tribution to science, and I did not deem it of suffi- 

 cient importance to quote authorities. Before putting 

 my compilation on paper, I had consulted Ferguson, 

 Silvanus Thompson, Ganot, Desehanel, some articles 

 by Dr. Atkinson of Chicago, and the article in 

 Science by Mr. H. W. Eaton. I cheerfully express my 

 obligation to all of these writers. Mr. Eaton's 

 article was specially helpful. As I claimed no 

 originality, there was no attempt or wish to deprive 

 him of any credit due. 



The greater part of Mr. Benjamin's pamphlet was 

 written by himself. W. Le C. Stevens. 



Brooklyn, Oct. 19. 



Recent Proceedings of Societies. 



Academy of natural sciences, Philadelphia. 



Botanical section, Oct. 12. — Mr. Aubrey H. Smith 

 described the flowering of Gordonia pubescens in 

 Bartram's garden and gave a history of the species. — 

 Mr. John Eedfield spoke of the topographical fea- 

 tures of Martha's vineyard and Nantucket, in con- 

 nection with the flora of those islands. The northern 

 part of the former rises into rounded gravelly hills 

 of considerable elevation, composed of gravel drift, 

 with occasional large bowlders. They are evidently 

 of glacial origin. The more central portion consists 

 of level plains of gravel covered with oak, mostly 

 Quercus obtusiloba. The general character of the 

 flora is much like that found on the summit of the 

 divides in southern New Jersey, though much more 

 limited as to species. Farther south, extensive 

 ponds both of fresh and salt water introduce theii' 

 characteristic vegetation. In Nantucket he had 

 found the gravelly hills of much less height, the 

 greater portion of the island consisting, in fact, of 

 treeless plains. One extensive grove of Pinus rigida 

 exists in the central portion, and is known to have 

 been planted. The most characteristic plants of the 

 plains seemed to be bear-berry, Arctostaphylos uva- 

 ursi. which grows there in great profusion. The two 

 species of Hudsonia abound, the Hericoides being 

 seen everywhere, and less frequently the bluish tufts 

 of H. tonientosa, Polygalae polygama, Myrica, 

 cerifera, and various vaccinae abound. He saw many 

 large patches of Corema Conradii, the existence of 

 which in Nantucket had first been made known by 

 Mrs. Owen of Springfield, Mass. But the most inter- 



