May i8, 1888.] 



SCIENCE. 



24 V 



The data on secular variation in rainfall, p. ig, Monthly Weather 

 Review for April, 1887, show, however, that, even with these errors 

 corrected, the rainfall at Leavenworth for the past twenty-five years 

 has been considerably greater than for the previous twenty-five 

 years. There is no doubt that material errors existed in the old 

 records, some of which are due to neglect or falsification of 

 records, while others, as in this case, are due to gross carelessness. 



Rainfall data are now being collated by the Signal Office with a 

 view to their examination and discussion ; but the more the records 

 are examined, the more possible it seems that observations prior to 

 1870 should be neglected, except in cases of well-known and reli- 

 able observers. A. W. Greely. 



Washington, D.C., May 12. 



Disparate Vision. 



Mr. Hyslop'S experiments in physiological optics as detailed in 

 Science, Nos. 261, 262, and 274, are interesting in that they show 

 the importance of monocular perceptions in attaining what we may 

 think to be binocular effects, even though they may not fully dis- 

 prove the generally accepted theory of corresponding retinal points. 

 Having devoted much time to this subject (see American Journal 

 of Science for November and December, 1881, March, April, May, 

 October, and November, 1882). I may perhaps claim some practice 

 in experiments of this' kind. The result of former investigations 

 was my total abandonment of the geometric considerations which 

 formed an integral part of Brewster's theory of binocular vision, and 

 which have been repeated time and again since his day. The 

 empiristic theory, as developed by Helmholtz, seems more consis- 

 tent with the more general theory of evolution now universally 

 accepted as fundamental in biology. According to this, we rap- 

 idly learn in infancy to interpret our binocular perceptions by 

 experience that is too complex for analysis. Assuming a certain 

 inherited structure for the retina, which is alike for the majority 

 of individuals of the race, it remains possible to modify our per- 

 ceptions slightly by training ; and it would not be safe to deny 

 that in exceptional cases binocular perceptions may result from 

 simultaneous impressions on retinal points that are decidedly dis- 

 parate. I have elsewhere adduced arguments to show that no 

 strictly mathematical interpretation can be put upon the theory of 

 corresponding points {American Journal of Science, May, 1882, 

 p. 355 ct seq). The perception of the third dimension in space 

 without any of the aids resulting from shading, comparison, or mo- 

 tion, has lately been shown to be quite possible with monocular 

 vision alone (American Jojirnal of Psychology, November, 1887, 

 p. 99, article on the Horopter, by Mrs. Franklin). I had no difficulty 

 in attaining this monocular perception in repeating Mrs. Franklin's 

 experiments. 



But although constrained to assign much greater potency to 

 monocular vision than was customary after the stereoscope became 

 generally known and used, and although our interpretation of bi- 

 nocular perception has to be much more elastic than it formerly 

 was, there seems to be not yet sufficient ground for the belief that 

 any large part of our binocular perceptions are the result of impres- 

 sion on pairs of retinal points that are widely disparate. The same 

 perception may be changed by force of will or of imagination, and 

 with various degrees of success by the same person at different times. 

 "Without denying the validity of Mr. Hyslop's perceptions, -I do not 

 succeed in getting exactly his results. Combining the two circles by 

 either convergent or divergent vision, the binocular effect is an ellipse 

 whose plane is perpendicular to the meridian plane only when their 

 inclinations to this plane are equal. This perception is rigidly bi- 

 nocular. Let, now, their inclinations be different. For example : let 

 the plane of the circle A make an angle of 30° with the meridian plane, 

 and B an angle of 60°, the two being seen by cross-vision. In the accom- 

 panying diagram the cards are supposed to be seen edgewise, the two 

 eyes being at R and L. The plane of the resultant ellipse changes 

 about to the position C ; the horizontal axis, which was previously 

 the shorter one, becoming now much longer than the vertical axis, 

 which has remained unchanged. The projection of the circle v4 on 

 the retina L is quite a narrow ellipse, while that of B on the retina 

 R is almost if not quite circular, the vertical diameters of these 

 ellipses being nearly equal. At the top and bottom of the resultant 

 ellipse the perception may be due to impression on corresponding 



retinal points, while for other parts the impression is on disparate 

 points. Very little attention is required to perceive the separate 

 monocular images. By still further diminishing the angle a and 

 increasing /3, a limit is reached at which binocular fusion ceases to 

 be possible. Two ellipses are seen, apparently crossing each other 

 in space about where C was ; the plane of one being nearly parallel 

 to A, and that of the other nearly parallel to B. By indirect mo- 

 nocular vision, A is still seen by the right eye, and B by the left. 

 The locality of the crossed ellipses is not so definite as was that of 

 the binocular ellipse ; but the illusion of suspension in space still 

 remains, and with it is the monocular perception of the third dimen- 

 sion in space. Even when a is very nearly equal to /3, it is pos- 

 sible by rivalry of retinal impressions to gain or lose monocular 

 perceptions alternately with binocular resultants. But the clear- 

 ness of the binocular illusions is more pronounced than that of the 

 monocular in proportion as the separation of the disparate points 

 impressed becomes less. It is fair to conclude that binocular vision 

 is at its best when there is perfect correspondence of at least a 

 goodly proportion of the retinal points impressed, and but slight sep- 

 aration of disparate points. But it is quite necessary, in the majority 

 of cases, that there shall be some such disparateness. The mental 

 effect produced is instantaneous. Since double images, whether 

 homonymous or heteronymous, are rarely ever perceived except as 



-'? 



the result of special ocular training, and since the binocular percep- 

 tion of depth in space may result where one element may, on geo- 

 metric grounds, be considered to be combined with other elements 

 so as to produce at the same instant both homonymous and hete- 

 ronymous double images {American Journal of Science, October, 

 1882, p. 5), binocular vision is far from being so simple and easy of 

 explanation as it seemed to the students of forty years ago: 



W. LeConte Stevens. 



Brooklyn, N.Y., May 5. . 



Agriculture and Late Quaternary Geology. 



In view of the effort now being made to endow the United States 

 Geological Survey with the means of carrying into effect the " classi- 

 fication of lands " called for in the act creating it, it may be of 

 interest to record one out of many instances where this classifica- 

 tion, in connection with agricultural phenomena, affords informa- 

 tion equally interesting to the geologist and the farmer. 



At a late visit to the upper San Joaquin valley for the purpose of 

 locating on a representative soil a culture experiment station under 

 the Hatch Act, the writer was under the necessity of obtaining a 

 cross-section of the great valley in the latitude of Tulare City, from 

 Lake Tulare on the west, to the foot-hills of the Sierra Nevada on 

 the east. 



The dark-tinted loam-deposits at present forming on the edge of 

 that lake being already familiar, it was easy to recognize in the 

 ' black-lands ' belt, that begins about two miles westward of the 



