Sept. 6, 1877] 



NATURE 



397 



unison with the sunspot periods, at all events enough to warran' 

 further and more complete investigations being made on the 

 point. I will not trespass on your valuable space further than 

 to add a similar rouijh indication of the inverse relation that 

 holds in the case of the summer rainfalls. They will be seen to 

 be greatest in years o( maximum sunspot. 



In the fullowinj; };roups the rainfall of Dehra is taken for the 

 months of June, July, August, September, and October during 

 which the south-west monsoon rains fjll. I give the separate 

 rainfall durmg these months for each year, as well as the totals 

 and averages. 



Average for each year 



1869 70'5 



1870 77-2 



1871 IOO'2 



247 '9 



Average for each year 71 "6 



1865 t 58-9 ) 



1866 I 67 -oV I 186-5 

 I 1867 I 6o-6) 



Average for each year 82 "6 Average for each year 62' i 



The averages give a mean average excess in each year in the 

 maximum sun-spot groups of 19 "9 inches over each year in the 

 minimum groups ; a gigantic difference certainly, and apparently 

 maintair>ed pretty con-istently throughout by the rainfall of each 

 year. The defect in the winter rainfall of years of maximum 

 sunspot and tlie corresponding excess in years of minimum sun- 

 spot together combine to render the excess in years of maximum 

 sun-spot less apparent in the total annual falls, though it still 

 exists to a certain extent. The present year (at present abnor- 

 mally deficient in solar activity) appears destined to fulfil the 

 preceding relations to an alarming extent. The winter rain- 

 fall was unusually plentiful throughout Northern India, while, 

 up to the present time, when the monsoon should be in full 

 swing, the rains, except in Eastern Bengal, have been so scanty 

 that unless rain falls soon and abundantly, we shall have to face 

 a famine as fearful as that which is just now devastating Madras. 



E. D. Archibald 



Greening of Oysters 



It has long been known that oysters, when removed from the 

 sea and kept artificially in shallow pits filled with salt-water, 

 assume a green colour which is or was much thought of by 

 epicures. While this spring at Le Croisic, at the mouth of the 

 Loire, my friend. Dr. Bornet, informed Prof. Lankester and 

 myself that this singular change was particularly observable in 

 the oyster preserves in the neighbourhood. He was at that 

 time at a loss for an explanation, but [ have just received a letter 

 from him in which he gives the solution of the problem, and this 

 will I think be so interesting to many of the readers of Nature, 

 that I have ventured, although without his permission, to 

 communicate it to them. 



"As a souvenir of our meeting at Croisic, I send you some 

 CoralUnacice from that locality. Several have not yet been 

 delected on the English coast, where, however, they ought to 

 occur. I have added a Diatom, Navicula ftisiformis, Grunow, 

 van ostreana. 'fhis species, whose contents are of a cobalt-blue 

 colour during life, occurs in profusion in the oyster-i^reserves of 

 Croisic, and it is because they feed on this Diatom that they 

 become green. Nothing is easier than to demonstrate the fact 

 by placmg white oysters in a plate of sea-water containing 

 nothing but Navicula fusiforinis, and the "greening" takes 

 place in thirty-six hours. As often as the experiment is repe.itei), 

 the same result follows. But why should Na-jiciila fusi/ormis 

 be blue, while all other Diatoms are colourless?" 



W. T. Thiselton Dyer 



P. S. — Since this note has been in type Prof. Oliver has 

 called my attenliun to a paper in the " Memoires de la Societe 

 Linneene du Calvados," 1S24, pp. 135-158, by Benjamin Gaillon, 

 *'.Sur la cause de la colcraiion des Huitrcs et sur les Ani- 

 malcules qui servent a leur nutrition." These animalcules form 

 masses which he compares to the green matter of Priestley, and 

 as he refers them to the genus Navicula of Bory, they are 



no doubt identical with those which Dr. Bornet has studied. 

 Gaillon refers to an earlier memoir of his own on the same 

 subject published by the Academy of Sciences of Rouen, and in 

 the Animlcs des Sciences Physiijues, for 1821. W. T. T. D. 



Reproduction by Conjugation 

 The phenomenon to which Mr. Bennett alludes is, I presume, 

 well known ; but it is not universal, though common. He will find 

 illustrations in Hassall's "British Freshwater Alg;e," where the 

 zygospores aie formed in both filiroents simultaneously; e.g., 

 plate 19, Zy«nema (Spirogyra) ; plate 38, several species of 

 Tyndaridta. But in those genera in which the Zygospore is 

 formed behveen the filaments it would seem impossible to decide 

 which is male and which is female, e.g., plate 39, Tyndaridea 

 cpnspicua, immersa, Ralfsii, and decussala ; or in the genera 

 Mewcarpus, plates 42-47, and Slenocarpus, plates 47-49 



George Henslow 



Strange Dream Phenomenon 

 After reading the interesting letter on a " Strange Dream 

 Phenomenon" which appeared in Nature (vol. xvi. p. 329) it 

 occurred to me that it might be wortli while to put on record 

 the following experience which connects in a very striking 

 manner the phenomena of dreaming and subjective vision. 

 S )me time ago, when rather tired by overwork, I dreamt during 

 the night that some one had entered my bedroom and was 

 approaching the pillow under my head with the intention of 

 abstracting some valuable papers which I fancied were concealed 

 beneath it. I noticed in every particular the dress, stature, and 

 features of the intending robber, but just as he put f>rward his 

 hand towards the bed I began to awake, slowly at first, but with 

 great celerity as soon as I perceived the figure of my dream 

 walking slowly down the side of the bed ; wide awake now, I 

 watched it reach the corner bedpust, turn round, and with 

 measured noiseless step pass along the foot, till on coming 

 between the window and myself it disappeared, as all the 

 "ghosts " with which I was then afflicted were wont to do when 

 shone through by the light. 



I did not sleep any more for the rest of the night, and hence am 

 perfectly certain that this was not "a dream within a dream," 

 but a clear case of a subjective vision prolonged from the sleep- 

 ng into the waking state, and thus aff irdin:^ evi'ience to pr<jve 

 the essential identity which underlies the phenomena of "dream- 

 ing dreams " and seeing " ghosts." W. J. S. 



OUR ASTRONOMICAL COLUMN 

 The Satellites of Mars.— The Paris correspondent 

 of the Times, writing on August 27, states that at the 

 meeting of the Academy of Sciences the same day, M. 

 Faye had announced trie independent discovery of the 

 satellites of Mars by M. Borrelly at Marseilles, claiming 

 (or him even an earlier detection than was effected at 

 Washington. Prof. Watson's name being introduced as 

 the American discoverer instead of that of Prof. Asaph 

 Hall, it is clear that the statement has arisen from a mis- 

 conception on the part of the reporter at the sitting of the 

 Academy, who has confounded the discovery of No. 174 

 of the minor-planet group, by Watson and Horrelly, with 

 that of the satellites of M ars. 



A letter from Rear-Admiral Rodgers, Superintendent 

 of the Naval Observatory, Washington, to the Secretary 

 of the Navy, dated August 21, furnishes particulars of the 

 observations and calculations bearing upon both satellites, 

 which had been made up to that date. The outer 

 satellite was remarked on the i ith, but its true character 

 was not certainly recognised until the i6th. On the 

 following night Prof. Hall first observed the inner 

 satellite. The discoveries were telegraphed to Messrs. 

 Alvan Clark and Sons, at Cambridgeport, on the iSth, 

 that confirmatory evidence of the existence of the satellites 

 might be obtained by means of the 26-inch telescope of 

 Mr. McCormick, at present in the hands of those eminent 

 opticians, who succeeded in verifying Prof. Hall's dis- 

 covery, as did also Prof. Pickering and his assistants at 

 Cambridge, Mass. On the 19th the discovery was 



