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NA TURE 



[June i i, iyo8 



YlSlWh ILLUSION AND FIXATION. 



A REMARKABLE new visual illusion is described by 

 Dr. James Eraser in the Journal of Psychology for 

 January. In the first form of the illusion a word (such 

 as " LIFE "} is printed in capital letters on a chequered 

 background of black, grey, and white squares. The double 

 outline of the letters is not traced in continuous lines, but 

 is constituted by a band consisting of short lines, alter- 

 nately black and white, slightly inclined to the direction 

 f)f the limbs of the letter. This band may conveniently 

 be regarded as representing a cord made of two strands, 

 .black and white, twisted together. In these circumstances 

 the letters appear, in general, to be inclined several degrees 

 from their actual directions, the sense of the deviation 

 varying with the direction of the constituent lines of the 



..♦.^♦.^ ♦ 







rillusory band (see Fig. i). .\ number of figures are given 

 illustrating variants of this form of the illusion, and, 

 facilitating a study of the limits within which it persists. 



In a second form of the experiment concentric circle, 

 ror ellipses made of the " twisted cord," and laid upon a 

 chequered surface of peculiar construction, are shown t.) 

 sufi'er apparent distortion of an extraordinary character 

 ((see Fig. 2). 



The paper (which is excellently illustrated) concludes 

 with a short discussion in which the author points out 

 .the factors which appear to him to determine the presence 

 and disappearance of the illusion. 



^Upright letters of appropriate size and concentric circles of appropriate 

 diameter, drawn on tracing paper and stiperimposed upon the figures, 

 will demonstrate the character of the illusi jn in each case. * 



The Psychological IJevicw has published, as a mono- 

 graph supplement (November, 1907), an account of an ex- 

 perimental study of visual fixation conducted by Prtjf. 

 Raymond Dodge in the psychological laboratories of 

 W esleyan University. The main result of this careful and 



interesting work is to render untenable the traditional 

 assumption of an identical anatomical and functional centre 

 of the retina, to which all visual processes are referred 

 or referable. I'rof. Dodge shows (following, in pan, 

 Delabarre and McAllister) that during supposed fixation 

 there is continuous movement of the point of regard over 

 a variable area of appreciable extent. This is the result 

 of uncompensated disturbances produced by the pulse and 

 respiration, and by irregular head and body movements. 

 On the other hand, the investigation has brought to light 

 a number of eye-movements compensatory of movements of 

 head and trunk, the perfection of which (since the reaction 

 time of the eye is relatively slow) points to the existence 

 of highly organised motor systems embracing both ocular 

 and somatic muscles. The discovery that there is, strictly, 

 not a fixation point, but merely a fixation area, renders 

 improbable the current hypothesis that assumes a tendency 

 to transfer every peripheral stimulus to the centre of the 

 fovea. .Actual experiment shows, in fact, that when a 

 given word is read the point of regard may rest indiffer- 

 ently in many situations. These observations have, 

 obviously, an important bearing upon the question "of 

 retinal space-perception, and Prof. Dodge uses them to 

 discredit the theory that the motor-impulse by which a 

 peripheral object of regard would be brought to a sup- 

 posed constant intra-foveal fi.xation point is a " local 

 sign " differentiating the retinal point stimulated from 

 every other. He proposes to substitute for it the concep- 

 tion of a differentiated organisation of the retinal groups, 

 and shows how such an organisation might be brought 

 about by the agency of the fixation movements. 



SOME RECENT PETROLOGICAL PAPERS. 



A DETAILED study of the granite of Bri.xen has been 

 ■^ contributed by Herr Bruno Sander to the Jahrbucli 

 der fc.fc. gcologischcn Rcichsanstalt, vol. Ivi. (1906), p. 

 707, which involves inany interesting questions of the 

 intimate penetration of sediments by igneous rocks, and 

 directs fresh attention to the marginal fades known as 

 tonalite-gneiss (pp. 726-34). The author shows that a 

 foliated structure existed in this rock before deformation 

 by pressure occurred. Dr. Trener, on the other hand 

 {ibid., pp. 415 and 458), in a paper on the Presanella 

 group, • containing many , petrographic details, treats 

 tonalite-gneiss as a product of pressure, and the basic 

 inclusions in it as segregations. One of Sander's most 

 suggestive observations is the finding of amphibolites, 

 closely resembling the tonalite-gneiss, in the old limestone 

 series that has been invaded by the granite ; and he is led 

 to ask (p. 734) whether the occurrence of tonalite-gneiss 

 does not in some way depend on the horizon selected by 

 the granite for its intrusion continuously from Meran to 

 Mauls. 



Dr. Trener's paper, just referred to. contains (p. 4S4) 

 a valuable appendix on graphite, in which Luzi's " graphi- 

 tite " and Sauer's " graphitoid " — a graphite in meta- 

 morphic rocks — are both opposed as mineral species. 



Mr. Joseph BarrcU's " Geology of the Marvsville Mining 

 District, Montana " (U.S. Geological Survey, Prof. Paper 

 No. 57, 1907) is also " a study of igneous intrusion and 

 contact mctamorphism," dealing particularly with a great 

 batholithic inflow of granite among pre-Cambrian strata 

 at the opening of Cainozoic times. \'arious igneous rocks, 

 from gabbros to aplites, appear as subsidiary intrusions. 

 The contact-phenomena have been studied to advantage in 

 mine-sections down as far as 1300 feet. .\ strong case is 

 made out for the occurrence of " stoping," by the falling 

 in of blocks " 10 to 200 feet or more in thickness and of 

 considerably greater length " (p. 172), and the consequent 

 rise of the granite into its former cover of Algonkian 

 rocks. 



Mr. R. .\. Daly, anotfier strong advocate of " stoping," 

 in a paper on the Okanagan composite batholith of the 

 Cascade Mountain system (Bull. Geol. Soc. -America, vol. 

 xvii., 1906, p. 320), urges that this batholilh actually re- 

 places an equal or approximately equal mass of the older 

 solid rock. Those of us who have been maintaining this 

 somewhat natural view from field-observations on bntho- 

 liths in our own islands must join with Mr. Daly in hail- 



NO. 2015, VOL. 78] 



