June i i, 1896] 



NA TURE 



125 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.] 



Tidal Migrations of Limpets. 



While spending a few days, in March of this year, at the 

 village of ilatadona, situated on the south-east coast of British 

 New Guinea, facing the China Straits and nearly opposite to 

 the large island of Rogeia, I had the opportunity of making 

 some observations on the habits of a species of limpets. 



On the beach near Matadona there is a sort of rugged plat- 

 forn\ formed by massive eruptive rocks, extending seawards and 

 presenting in some places a more or less vertical frontage, of 

 some three feet in height, to the sea. 



The rocky platform, covered at high tide, is quite exposed at 

 low tide, the sea receding a considerable distance away from it. 



(Ireat numbers of limpets live on the sub-vertical front of the 

 rficks, while the numerous small holes and crevices, with which 

 it is riddled, are occupied by Chitons, several of which are often 

 crowded together in a very limited space. 



Several species of Patella inhabit these rocks at various points, 

 but as a rule they occur as isolated individuals. One species, 

 however, occurs in large herds of a hundred or so individuals, 

 and it is to this gregarious Patella that the following notes refer. 



At low tide these limpets are attached to the seaward face of 

 the rock, quite at its base, adjoining the sand of the beach, and 

 it may then be observed that the zone of limpets, as a general 

 rule, occupies a lower level than that of the Chitons. 



It may also be noted that many of the limpet shells are them- 

 selves coated with Nulli pores and other marine plants. 



I have several times observed, at the time of flood-tide, that 

 at the approach of the surf, when the latter gets so close as to 

 spray the rocks, the limpets commence to crawl slowly up the 

 face of the rock, and as the tide rises higher so they climb 

 higher, always keeping above the level of the surf 



It can often be observed that they progress in the form of a 

 triangle, the leader at the apex. 



From the time the procession commences until they reach the 

 summit of the rocks, out of the reach of the violence of the 

 surf, the slow movement is practically continuous, the whole 

 company of limpets being found on close inspection to be in 

 motion, and producing a unique effect. 



The Chitons, sheltered in their nooks and crannies, undertake 

 no such migration: so that, in general, the zone of limpets is 

 above that of the Chitons at high tide. 



When the ebb-tide sets in, the limpets start on their return 

 journey; but I have not actually assisted at the downward pro- 

 cession. Between the tides they are stationary, but they produce 

 no scar on the rocks, so that there can be no question as to 

 their " homing " on the same spot. 



On returning to the rocks on one occasion, after a stiff' south- 

 easterly breeze, I found the sand banked up to the depth of 

 some two feet against the face of the rocks, approximately up 

 to the level of the zone of Chitons, some of the latter being 

 actually buried beneath the surface of the sand. Others again 

 of the lower lying Chitons had shifted their positions in con- 

 sequence of the inroad of sand. 



None of the limpets were thus buried, and they occupied 

 their usual relative position at the base of the available face of 

 rock. The zones of limpets and Chitons then nearly coincided. 



This tidal migration of limpets is interesting in comparison 

 with the periodical phenomena in the lives of other marine 

 organisms ; while the elevation of the limpet zone through the 

 formation of a sandbank, may perhaps suggest stratigraphical 

 reflections. Arthvr WiLi-iiv. 



Sydney, April 22. 



Butterflies and Hybernation. 

 In connection with Mr. Pidgeon's communication, under 

 the above heading, in N'ATfRE of April 2, respecting the 

 probable wintering of a tortoiseshell butterfly in a bath-room, 

 I may state that the hybernation i>f butterflies is of well- 

 established occurrence in at least certain portions of South 

 Africa, where one species in particular, namely. Precis sesamiis, 



NO. 1389, VOL. 54] 



Trimen, assembles in numbers at the end o. the summer 

 season for the purpose. This very distinct dingy blue and red 

 insect is plentifully distributed in East Griqualand and Natal, 

 especially affecting the road-cuttings between Ixopo and the 

 Ingeli-Zuurberg mountain chain. As remarked in Mr. Roland 

 Trimen's monograph on .South African butterflies, it likes 

 shady places under a roadside bank or rocks in a cutting ; and 

 Colonel Bowker — an enthusiastic and renowed South African 

 lepidopterist — is quoted as having seen them congregated under 

 rocks and in holes of dry banks, as many as twenty-nine being 

 captured by placing the net over them. Their dark bronzy 

 green under-colouring renders them, when thus massed, almost 

 inconspicuous in association with w-ithered fern, grass, &c., and 

 it is only by startling them that one very often becomes aware 

 of their presence. I particularly call to mind, while on one of 

 my botanical rambles in the Lower Umzimkulu district of East 

 Griqualand in 1S85, accompanied by a younger son of Mr. 

 Donald Strachan, unexpectedly flushing at least fifty of these 

 butterflies in the cold frosty season of Tulyi in a secluded glen 

 of the \'ubugas rivulet. Upon a little searching among the 

 scrub and bush we discovered a boulder, under which there 

 must have been as many again, if not more. These we roused 

 out with branchlets, some being more torpid than others ; but, 

 as we retired from the spot, they all flitted back to their trysting- 

 place. This was at the severest time of the season, and I never 

 doubted, after having observed the massing of this butterfly at 

 all times during the winter, that it emerged safe and strong in 

 the ensuing spring. A description and coloured figure are given 

 in Mr. Trimen's work, vol. i. p. 231, pi. iv. I. 3. 



Cape Town, May 20. W. Tyson. 



Becquerel's Colour Photographs. 



I SEE that the photographs in colour, taken by Becquerel's 

 plan, are said to be mainly due to interference. My own observ- 

 ations do not confirm this statement. A photograph of the 

 spectrum in colours can be readily taken on silver chloride on a 

 glass plate, and be examined both by reflected and transmitted 

 light. The colours in the two cases are identical, which is con- 

 trary to the "interference" explanation. 



W. DE W. Abney. 



Bolton Gardens South, S.W. 



Cannizzaro Memorial. 



Since my return from Italy, I have been so frequently asked 

 by friends and admirers of Prof. Cannizzaro what form it is 

 proposed to give to this memorial, that I wish, through your 

 valuable medium, to make it known that it is intended to present 

 the Professor with a medal commemorative of the occasion, and 

 to devote the balance of the sums subscribed to the creation of 

 a Cannizzaro prize or medal to perpetuate his memory, the 

 details of which will be left in his hands. 



LUDWIG MOND. 



Rontgen Ray Experiments. 



It has been generally noticed that when focus tubes become 

 much blackened, presumably by volatilisation and deposition 

 upon the glass of the platinum of the anode, they cease to be 

 eft'ective owing to the apparent increase in their internal resist- 

 ance. This is generally attributed to increase in the vacuum due to 

 the occlusion of the residual gas by the platinum black. This 

 may in part be the true explanation, but another is to be found 

 in a curious phenomenon discovered by Prof. Crookes, and 

 described in his 1 89 1 presidential address to the Institution of 

 Electrical Engineers. He says : " It appears that the greater 

 the phosphorescing power of the substance surrounding the 

 poles, so much easier does the induction spark pass. Surround 

 the poles with Bohemian glass or Yttria — two phosphorescent 

 non-conductors of electricity— and the induction spark passes 

 easily : immediately I surround the terminals with a non- 

 phosphorescent conductor" [a film of deposited silver] |'the 

 current refuses to pass." Very possibly the deposited platinum 

 in an old or overworked focus tube has a similar eff'ect to the 

 silver in Prof. Crookes' experiment. I have recently had ex- 

 perience with a tube of special form which was much blackened, 



