June 28, 1877] 



NATURE 



16' 



only one glacial period and that the disappearance of palaeolithic 

 man from Northern Em'ope was principally due to the sub- 

 mersion of the greater part of the land beneath the water of an 

 immense freshwater lake or sea, at or a little before the culmina- 

 tion of the ice age. If Mr. Geikie's views should be ultimately 

 accepted, the term " inter-glacial " will be most appropriate ; 

 but should, as I hope and believe, mine be proved to be nearer 

 the truth, I should prefer to use the term " pre-diiuvial " instead 

 of "pre-glacial," as heretofore, to express the age of palasolithic 

 man. Thomas Belt 



The Cedars, Ealing, June 22 



Will you kindly allow me to correct an apparent breach of 

 official etiquette and act of discourtesy in my last week's letter? 

 I should have said that oniy two geologists prominently interested 

 in the question at issue had seen my evidence ; for, of course, 

 Mr. H. W. Bristovv, F.R.S., Director of the Geological Survey 

 of England and Wales, has been kept fully en rapport with my 

 work, and has several times visited me at Brandon. I am 

 anxious that no statement of mine should appear to slight so 

 eminent a geologist and so considerate a friend. 



Brandon Sydney B. J. Skertchly 



Colour-Sense in Birds — Blue and Yellow Crocuses 



Unless your readers are quite tired of the subject, may I add 

 a fact which will be subversive of a good deal that has been 

 written about yellow crocuses and sparrows. I dislike yellow 

 crocuses, and four seasons since planted some hundreds of blue 

 and white in the garden underneath my windows. For two 

 seasons they fluwered in beautiful prolusion. In 1876 the 

 sparrows for the first time dt-stroyed these flowers completely. 

 I allowed the roots to remain for another year — viz., 1877 — but 

 they suffered the same usage, hardly a single flower being left 

 uninjured. .So complete was their destruction that I have had 

 the roots dut; up! 



I regard the proceeding as an imitative one ; blue and white 

 crocuses, not being common in the vicinity, were new to the 

 sparrows, and until one more experimental than the rest attacked 

 them they were safe. 



A similar result will occur with domestic pigeons ; if reared 

 exclusively with small grain, as wheat and barley, they will starve 

 before eating beans. But where they are thus hungry, put a 

 bean-eating pigeon amongst them, and the habit is immediately 

 propagated. 



I have seen fowls refuse maize at first, but on seeing others 

 eat it, they follow suit, and become excessively fond of it. 



W. B. Tegetmeier 



Purple Verbenas 



Having now read for the first time the letters in Nature 

 regarding the preference that sparrows show for the yellow 

 crocus, it might perhaps help to elucidate the problem were it 

 known that tne choice of colour is not only confined to birds, as 

 a few years ago our garden was infested by rabbits and there 

 was a row of eight beds planted in turn, with white, red, and 

 purple verbenas. The flowers of the red and white were eaten 

 close off, whilst those of the purple were never touched. This 

 happened three years running, since which, the garden, being 

 protected by wire netting, has remained undamaged. 



A. M. Darby 



Japanese Mirrors 



Your correspondents, Messrs. Atkinson, Highley, and Darbi- 

 shire, have referred to several conjectures and experiments re- 

 specting the curious Japanese mirrors and the patterns they 

 reflect. None of these gentlemen have, however, referred to the 

 suggestion offered by Sir David Brewster in the Philoiophical 

 Maga-Jne for December, 1832. In this paper Sir David drew 

 attention to some similar phenomena in the light reflected from 

 the surfaces of burnished buttons of metal, arguing that in the 

 mirrors (of which at that time he apparently had seen no actual 

 specimen) there were slight actual inequalities of surface, artificially 

 produced, but concealed from observation by their slightness of 

 depth and by the brightness of the poUsh. This, of course, may 



be independent of the particular figures raised in relief on the 

 back, as in the case cited by Mr. Darbishire ; and so thought 

 Sir David, for he added : — 



"Like all other conjurers, the artist has contrived to make the 

 observer deceive himself. The stamped figures on the back are 

 used for this purpose. The spectrum in the luminous area is not 

 an linage of the figures on the back. The figures are a copy of 

 the picture which the artist has drawn on the face of the mirror, 

 and so concealed by polishing that it is invisible in ordinary 

 lights, and can be brought out only in the sun's rays." 



I trust Mr. Atkinson may be able to learn in Japan the real 

 process of manufacture of these curious toys. Meanwhile are 

 there not specimens in many of our museums that would repay 

 examination ? Were there not some amongst lastyear's exhibits 

 at the Loan Collection of Scientific Apparatus ? 



Sylvanus p. Thompson 



University College, Bristol, June 25 



NOTE ON THE ELECTRICAL DISTURBANCE 

 WHICH ACCOMPANIES THE EXCITATION 

 OF THE STIGMA OF MIMULUS LUTEUS 



l\/r ANY years ago my attention was drawn to the 

 ^'-L excito-contractility exhibited by the lipped stigma 

 of Mimulus luteus, the structure of which I then gave an 

 account of in the Proceedings of the Edinburgh Botanical 

 Society. In connection with my recent investigation of 

 the excitatory variation in Dioiiaa I have, during the last 

 few weeks, in co-o^ieration with Mr. Page, mide experi- 

 ments for the purpose of ascertaining whether in this 

 organ, as in the leaf of Dioncsa, the change of form 

 provoked by mechanical stimulation is accompanied by a 

 similar electrical disturbance. 



Mimulus luteus is a favourite window plant on account 

 of its showy flowers and the facility with which it can be 

 cultivated. The mechanism of the contraction of the 

 stigma can be best studied in the inferior of the two 

 lobes, of similar size and form, of which the organ con- 

 sists. In the unexcited state, when 'the flower is in full 

 bloom, this lobe is curled outwards. The curling outwards 

 is due, as I long ago observed, to the turgidiiy of the 

 layer of loosely connected conducting cells, ending in 

 papillae, which constitute the stigmatic surface. So long 

 as this tissue is turgid the elastic lamina by which it is 

 backed is prevented from straightening itself, so that the 

 whole lobe forms a scroll of which the axis is transverse. 

 The elTect of touching any part of the lobe, and par- 

 ticularly the papillary surface, is to diminish the turgidity 

 of the tissue, as the result of which the organ slowly 

 expands so as to face and ultimately meet its fellow. 



The excitatory change of form which I have described 

 is, as in the case of Dioiicea, associated with an electrical 

 disturbance of which the following are the most important 

 features : — (i) The sign of the variation is the same as 

 in Dionaa, the excited structure becomes negative to 

 the rest of the plant. (2) The extent of variation is some- 

 what less than in Dioiicea, the electromotive force deve- 

 loped between the stigma and style being usually about 

 25-thousandths of a Daniell, whereas in Dioncea the 

 variation may amount to from 40- to 50-thousandths. 

 (3) The variation is of relatively long duration ; it reaches 

 its maximum at the ordinary temperature of summer, 

 about five seconds after excitation. It subsides at first 

 rapidly, then very gradually, so that the effect may not 

 have entirely passed off until two or three minutes have 

 elapsed. 



As in Dioncsa, the period of electrical disturbance is 

 shortened by increase of temperature. Thus in five 

 stigmas in which the period was measured at 20° C. 

 (68^ Fahr.) and at 37° C. (98" Fahr), the mean duration 

 of the interval of time between the commencement of 

 the electrical disturbance and the moment at which it 

 began to subside was 6'2 sec. at the higher temperature, 

 and 3 sec. at the lower. 



