February 21, 1896.] 



SCIENCE. 



275 



of the one eye with which they are looked 

 at)* are taken by us to be vertical lines. 

 This illusion is illustrated in Fig. 1. The 

 lines of the figure are all drawn through 

 a common point about three inches beyond 

 the corner of the paper. If one eye be put 

 in the position of this point (the other be- 

 ing closed), and if the paper be held hori- 

 zontally about on the level of the eye, the 

 lines will all seem to stand upright. The 

 reason is that when one eye only is used, 

 we have very small ground for knowing how 

 such a line is situated in the plane deter- 

 mined by it and by the nodal point of the 

 eye, and hence we take it to be a vertical 

 line faute de mieux, because by far the great- 

 est number of lines which strike the retina 

 in this meridian are vertical lines. With 

 many lines, the illusion is stronger than with 

 one, because every group of vertical staves 

 that we have ever seen has looked like this, 

 and it has probably never happened to us 

 to see a group of lines lying on the ground 

 in just this position. 



That this is the correct explanation of 

 the phenomenon of the lights is confirmed 

 by the fact that, upon looking at the reflec- 



tion with the head inclined through an angle 

 of ninty degrees, the illusion wholly dis- 

 appears. One can no longer believe that it 

 is possible to see the stream of light other- 

 wise than as lying flat upon the surface of 

 the lake. In this case the image of the line 

 of light falls along the eyes, from one to 

 the other, or just as a line would do which 

 went from right to left if the head were in 

 its normal position. Such a line we have 

 no tendency to see vertical, and hence we 

 now see the streak of light where it really 

 is on the surface of the water. With the 

 head wholly inverted, the line becomes 

 vertical again, but less strongly so than 

 when the head is in the customary attitude. 

 Chr. Ladd Franklin. 



*A'm. Jour: of Psychology, I., 101 and James' Prin- 

 ciples of Psychology, II., 95. 



CUBBENT NOTES ON PHYSIOGBAPHY. 

 THE TERTIARY PENEPLAIN IN MISSOURI. 



The prevalent opinion that the ' moun- 

 tains ' of the dissected Ozark plateau in 

 Missouri are old geographical features 

 meets welcome contradiction in an essay 

 by Keyes, State Geologist (Missouri Geol. 

 Survey, viii., 1894, 317-352). The rela- 

 tively even upland surface of the plateau is 

 explained as a peneplain of denudation ; 

 and the dome-like form of the plateau to- 

 day is regarded as the result of elevation 

 since the close of the Tertiary. The gen- 

 eral upland plain is dissected by steep-sided 

 or canyon-like trenches, in which the pro- 

 cess of deepening is still continued. " The 

 last elevation is not yet ended, and th& 

 changes of level in the region are probably 

 going on now as rapidly as they ever have 

 in the past geological time" (p. 352). 

 While the strata are nearly horizontal in 

 the Ozark plateau, they are tilted in the 

 Ouachita mountains, south of the broad val- 

 ley of the Arkansas river, in the State of 

 that name. Keyes regards the relatively 

 even crest lines of the Ouachita ridges as 

 representing the same peneplain as that of 

 the Ozarks; the broad valley of the Ar- 



