October 2i, 1920] 



NATURE 



243 



be io6j and of spotted dogfish io6S; the latter sink 

 quite fast when they stop swimming for a moment. 

 Presumably a basking shark would have a similar 

 specific gravity, yet it floats. To catch one and solve 

 the problem is not easy, but a friend kindly harpooned 

 a sunfish for me. It was 3 ft. long and weighed 

 1 15 lb. The body was completely enclosed in a rigid 

 case of some tough tissue resembling cartilage to the 

 naked eye, but extraordinarily light. This shell 

 varied from 075 to 1-25 in. in thickness except about 

 the head, where it was very massive. It could be 

 dissected off in slabs, and the thicker of these floated 

 when thrown overboard. With the absence of bone, 

 an immense fatty liver, and this queer, buoyant 

 cuirass, I think we have the solution in the case of 

 the sunflsh. G. C. C. Damant. 



H.M. Salvage Ship Racer, Portsmouth. 



Old Irish Maps. 



The fact that I have been able to refer your 

 reviewer to an important map with which he was 

 previously unacquainted is some compensation for the 

 necessity for his last sentence (Nature, October 7, 

 p. 180).' 



.\ few years ago I spent some weeks among the 

 maps in the library of the Geological Society of 

 London, and tried to settle the question of the dates 

 and editions of Griffith's maps. Since your issue of 

 October 7 I have re-examined the evidence, with the 

 additional help of a volume of Dublin addresses, once 

 the property of Prof. Phillips, which I have recently 

 obtained. The only conclusion I have been able to 

 arrive at is that the Irish and English literature on 

 the -subject is vague and contradictory. 



The writer of Sir Richard Griflfith's obituary in the 

 Geological Magazine, 1878, p. 525, states : "So long 

 ago as the year 1812 the first outlines were attempted 

 of ... a geological map of Ireland. No labour 

 seemed to Grifllth too great in order to carry out this 

 great work satisfactorily. Four editions of it were 

 published, the latest of which was issued in 1854." 

 |ud<l {loc. cit., ifk)!i, p. i.jq) tells us the large map of 

 Ireland was exhibited in 1838 and published in 

 March, 1839, and a second edition was published in 

 >«55- 



On June 13, 1830, Griffith read a paper (Journ. 

 Ciiol. Soc. Dublin, 1839, p. 78) on "Presenting to the 

 .Society the Geological Map of Ireland in the Large 

 Scale, the Result of my I.abours for Upwards of 

 Thirty Years." Later (loc. cit., 1857, p. 294) he read 

 a similar paper on " Preparing the Last Edition of mv 

 Geological Map of Ireland dated .April, 1855." Close 

 (Journ. Royal Geol. Soc. Irelaml, 1879. p. 141) stated: 

 "Very shortly after that [.April, 1838], in the same 

 year, the large map . . . was brought out, though for 

 some reason which does not appear it was not regu- 

 larly published so as to be accessible to all until 

 March 28, 1839, the date which is inscribed upon It"; 

 and (p. 142) : " In June, 1840, only fifteen months 

 after the last-mentioned edition, a new issue appeared. 

 ... In the short time menllone<l changes had been 

 made in the map in no less than forty places." We 

 also learn (pp. 144-45) that a small' edition of the 

 map was published, and in 1855 a revised and the 

 last edition was issued. 



.\pjohn (loc. cit., 1841, pp. 158-59) states that aft 

 early as February, 1841, "in point of f.nct, three maps 

 have been published by Mr. Griflfith, first, n map on 

 a comparatively small scale . . . and subsequently a 

 first and second edition of his large map. We have 

 alre.ndv seen how great are the discrepancies between 

 the two larger maps " — this, be It noted, being before 

 the publication of the 1853 and 1855 editions. 



NO. 2660. VOL. 106] 



The Geological Society possesses a large map 

 "revised in 1853," and a smaller map (circa i860) by 

 Griffith is "copied from the large map of 1853." 



So among these conflicting contemporary statements 

 as to dates and editions I may be pardoned, after this 

 lapse of time, for requiring to be "corrected." 



.Museum, Hull. T. Sheppard. 



A Visual Illusion. 



Mr. Turner should have consulted some standard 

 work on experimental psychology before claiming a 

 visual illusion as "new" (Nature, October 7, p. 180) 

 and advancing an explanation which experiment has 

 shown to be wholly inadequate to account for the 

 retinal after-sensations of movement. The effects are 

 quite independent of movements of the eyes, and 

 as truly " sensory " in character as the after-effects of 

 colour and brightness. C. S. Myers. 



Gonville and Caius College, Cambridge, 

 October 17. 



The phenomenon referred to by Mr. Turner and by 

 Prof. Boycott in Nature of October 7 and 14, pp. 180 

 and 213, was described by .Aristotle in his treatise on 

 dreams ("Parva Naturatia") thus: ".Also, the senses 

 are affected in this way when we turn quickly from 

 objects in motion, e.g. from looking at a river, and 

 especially from looking at swiftly flowing streams. 

 For objects at rest then seem to be in motion." 



The phenomenon has since been rediscovered times 

 out of number, e.g. by Purkinje in 1825, R. .Adams 

 in 1834, Johannes .Miiller in 1840, Sir David 

 Brewster in 1845, etc. I reinvestigated it experi- 

 mentally, and published the results of this research, 

 together with a historical survey, as a monograph, 

 "On the .After-Effect of Seen .Movement," in the 

 British Journal of Psychology (Monograph Supple- 

 ments, No. I, Cambridge University Press). 



A. Wohlgemuth. 



70 West End Lane, London, N.W.6, 

 October 18. 



The visual illusion described by Mr. Turner and by 

 Prof. Boycott in Nature of October 7 and 14 was 

 described by me in Nature of October 18, 1917 (vol. c, 

 p. 126), and commented on by Dr. F. J. Allen and 

 others on pp. 146, 165, 225, and 325 of the same 

 volume. It had also been described in Nature, 

 vol. Ixx., p. 107, and vol. Ixxviii., pp. 225, 277, and 

 305. It was also pointed out by me in vol. c, p. 284^ 

 that the phenomenon had been fully described by 

 Dr. John .Aitken in the Journal of Anatomy and 

 Physiology, vol. xiii., p. 322. The illusion may, per- 

 haps, be best seen by looking through a microscope 

 and slowly rotating the stage ; as soon as the rotation 

 is stopped the field appears to be revolving in the 

 opposite direction, and so strong is the illusion that 

 the stage may again be rotated very slowly in 

 the original direction for 10° or 15° and will 

 appear to the eye to be perfectly still. The same 

 phenomenon may be seen when a pianola roll is 

 stopped, the roll appearing to be slowly moving back- 

 wards. In some forms of pianola there is, in front 

 of the record, a glass panel on which is a small knob 

 for opening and shutting the panel; if the finger is 

 placed lightly on this knob while the roll appears 

 to be running back, I have the very curious tactual 

 illusion th;it the knob is also moving upwards, and 

 that it presses more and more against the finger. 

 One or two others, however, with whom I have tried 

 the experiment do not perceive the tactual illusion. 



C. J. P. Cavk. 



Ditcham Park, Petersfield, October 17. 



