June 25, 1903] 



NATURE 



77 



Phenomena of Visioii. 



Your correspondent, Mr. W. Betz, refers in his letter 

 of May 7 to the fact that an object just screened from 

 direct "vision by the nose (or by any other obstruction) 

 becomes visible if we rotate the eye in a direction away 

 from the object. This is a well-known phenomenon, and 

 a very interesting one on account of the curious facts with 

 regard to vision that can be deduced from it ; but it is not 

 in any way due to spherical aberration. It is rather a 

 perspective effect, being caused simply by the shifting of 

 the point of sight, which, being situated near the crystal- 

 line .lens, moves laterally as the eye is rotated about its 

 centre. There are several ways of demonstrating the 

 movement of the point of sight, but the experiment de- 

 scribed by Mr. Betz is perhaps the most convincing. 



An interesting corollary of this experiment is the generally 

 unfamiliar fact that we employ, two points of sight simul- 

 taneously in the act of vision, though we may use one 

 eye alone. Speaking generally, the eye wanders over any 

 object we may be examining with slight pauses at each 

 point of interest. Only at each pause do we really see, 

 and our final mental impression of the whole object may be 

 described as a mentally combined image of a series of 

 " snap-shots." The retinal image produced at each " snap- 

 shot " is a perspective view with the node of the crystalline 

 lens as the station point. The final mental picture is, how- 

 ever, a view with the centre of the eyeball (or its centre 

 of rotation) as a station point. This latter view is smaller 

 than the other, by reason of the fact that the centre of 

 rotation is some little way behind the crystalline lens, hence 

 the apparent size of an object varies as we study it. The 

 general effect is further complicated by the lateral move- 

 ment of the crystalline lens, which causes each momentary 

 snapshot to be taken from a different station point ; also the 

 final impression is more or less influenced by the impression 

 gained during the last fixed glance. Therefore we may 

 conclude, with a considerable amount of reason, that we do 

 not see objects exactly as they are. Really we only see 

 a combination of a number of views taken from different 

 points, and to arrive at a true understanding of what we 

 see we must employ our capacity of reasoning. The extra- 

 ordinary complexity of our mental visual conception is often 

 deceptive, though unconsciously so to many people who 

 have no idea of the peculiarities of vision. 



The various effects of the employment of the two station 

 points are not likely to be appreciated unless looked for, 

 but once you realise the fact, evidence is easily collected. 

 One of the most striking effects is the apparent movement 

 of fixed points. In the experiment described by Mr. Betz 

 the object point seems to play hide and seek with you, 

 popping out from behind the screen when you look in 

 another direction, and dodging back again when you try 

 to look straight at it. It seems to move with the eye, but 

 this effect is due mainly to the presence of the screen, for 

 under other conditions it will generally be found that the 

 apparent movement is -opposite to that of the eye. The 

 following experiment illustrates both the illusion of move- 

 ment and also the dependence of apparent size upon the 

 direction of vision. 



Place two objects at different distances from the eye and 

 ibtending a moderate angle at the eye so that both can be 

 -on when one is directly looked at. Look fixedly at one 

 object and estimate the distance between the two. Then 

 traverse the eye slowly on to the other object, and the dis- 

 tance between them most distinctly alters, the effect being 

 apparently due to a shifting of one or both of the objects. 

 You will find it somewhat puzzling to account for all the 

 various effects of movement that can be detected under 

 different conditions, but if you take all factors into con- 

 sideration, you will, I think, eventually find that the shift- 

 ing of the station point is primarily responsible for all the 

 effects produced; other than those due to spherical aberra- 

 tion. C. Welborne Piper. 

 May 15. 



Mr. Piper's explanation of the curious phenomenon 

 pointed out by Mr. Betz is presumably correct ; that there 

 are two station points used in vision can, however, scarcely 

 be demonstrated. The positions of the nodal points of 

 the eye are shifted during accommodation for near vision, 

 and perhaps this displacement is what Mr. Piper refers to. 



NO. 1756, VOL. 68] 



In general it is difficult to observe the apparent niotions 

 of objects which Mr. Piper mentions, and it is still more 

 difficult to trace such motions to the optical properties of 

 the eye, since we are here dealing, not with optical images 

 which can be directly examined, but with mental im- 

 pressions* Thus Konig pointed out {Wied. Ann., xxviii. pp." 

 367-368', 1886; " Gesammelte Abhandlungen zur physio- 

 logischen Optik," xiii. p. 58, Leipzig) that patients, on 

 first being provided with strong divergent spectacles, corn- 

 plain that, on moving their eyes without turning their 

 heads, stationary objects appear to move. After a time 

 this apparent motion ceases to be observed, or, indeed, to 

 be observable, but on removing thfe spectacles stationary 

 objects appear to move in a sense opposite to that previously 

 observed with the spectacles. In this case a readjustment 

 of judgment respecting visual impressions has been effected ; 

 the result shows to what a great extent judgment enters 

 into the act of vision. Edwin Edser. 



June 13. 



School Geometry Reform. 



In the unsigned review of Prof, barrel! 's " Elementary 

 Geometry " appearing in the issue of June 18, the following 

 sentence occurs: — "A feature to be noticed is that the 

 author gives three meanings of a plane angle, in the last 

 of which the angle is regarded as the plane space swept' 

 out by a line of indefinite length (one way) turning about 

 one end." It is unsafe to say that such a definition is 

 wrong, but it is certainly most undesirable in a school 

 book. The apprehension of the true nature of an angle is 

 one of the greatest difficulties that the beginner has to 

 encounter, and the way is not smoothed by the introduc- 

 tion of the idea of an infinitely extended space. It is 

 true, as Mr. Russell points out (" Principles of Mathe^ 

 matics," p. 416), that the definition can be made logically 

 satisfactory if the axiom of the whole, being greater than 

 its part, be rejected ; but this is an intolerable objection. 

 The best course for an educational book is that adopted by 

 Ronch^ and De Comberousse (" Trait6 de Geometrie," 

 189 1, p. 5), who say : — " La consideration de deux droites 

 qui se rencontrent conduit k une id^e nouvelle, qui est celle 

 A'incUnaison mutuelle ou d^angle, et qui, comme rid6e 

 de longueur, ne saurait 6tre d^finie, c'est-a-dire ramen^e k 

 une id6e plus simple." R. W. H. T. Hudson. 



June 22. 



RECENT EXCAVATIONS AT NIPPUR. 



IT was in 1884, at a meeting of the American Oriental 

 Society, that the first plans of an expedition to 

 Southern Babylonia were projected, and from that year 

 dates the beginning of the systematic scientific work 

 which is being carried on by the Americans at the 

 mounds of Nuffar, the ancient Nippur, with all possible 

 thoroughness. Since the year 1888, there have been 

 four expeditions sent out to excavate this ancient site, 

 and there is still much to be done there. The first 

 resulted in the discovery of a Parthian palace, and 

 many " finds " from systematic diggings in the 

 Temple of Bel, the cuneiform tablets alone numbering 

 two thousand; but ill-luck overtook the members of 

 the party, and, owing to trouble with the Arabs, the 

 camp was burnt and they themselves were robbed. 

 However, the next year, on reopening the works, there 

 was no opposition, and the labours of the expedition 

 were rewarded with eight thousand tablets of the 

 second and third millennium B.C., and in the third 

 campaign many pre-Sargonic ruins were discovered, 

 besides more than twenty thousand tablets. The last 

 expedition, which came to an end in 1900, was the 

 most successful of all ; the Parthian palace was com- 

 pletely explored, and, what was more important, the 

 great library of the Temple of Bel was located, and 

 twenty-three thousand clay tablets were excavated 

 therefrom, thus bringing the total number found up 

 to more than fifty thousand. 



