70 



SCIENCE. 



[N. S. Vol. IX. No. 211. 



fifty years, dwelling particularly on the work 

 accomplished by Leidy, Goddard, Neill, Hyrtl 

 and Gibbons Hunt before the recent improve- 

 ments in methods and instruments were heard 

 of. The communication was illustrated by a 

 large number of instruments and slides and was 

 fully discussed by Messrs. Goldsmith, Keeley, 

 Calvert and Dixon. 



Papers under the following titles have re- 

 cently been presented for publication: 



Some Cuban Species of Cerion. By H. A. 

 Pilsbry and E. G. Vauatta. 



Notes ou the Growth of the Hobble-bush, 

 Viburnum lantanoides. By Ida A. Keller. 



The Occurrence of Marcasite in the Karitan 

 Formation. By S. H. Hamilton. 



Margarita Sharpii, a new Alaskan Gastropod. 

 By H. A. Pilsbry. 



The Bone-Cave at Port Kennedy, Pennsyl- 

 vania, and its partial examination in 1894, 

 1895 and 1896. By Henry C. Mercer. 



Observations on the Classification of Birds. 

 By Dr. K. W. Shufeldt. 



A Study of the Type Specimens of Birds in 

 the Collection of the Academy, with a brief 

 history of the Collection. By Witmer Stone. 



Mr. Mercer's paper will be published in the 

 Journal of the Academy, the others in the Pro- 

 ceedings. 



E. J. Nolan, 



Secretary. 



DISCUSSION AND CORRESPONDENCE. 



THE SENSATION OF MOTION AND ITS EEVEESAL. 



To THE Editor of Science : The writer 

 has for a number of years noticed, during rail- 

 way journeys, a very peculiar reversal of sensa- 

 tions of motion received through the eye, of 

 which he has never seen any description or ex- 

 planation. The following description and ex- 

 planation may, therefore, interest the readers of 

 Science. A sensation of reversed motion of 

 stationary points in the field of vision is per- 

 ceived by the writer after gazing fixedly out of 

 a car window at a moving landscape. This 

 sensation is quite intense when the eyes are 

 first turned away from the window, dies away 

 gradually, and is greatly weakened by atten- 

 tive vision. For example, when looking out of 

 the rear door of the train the various objects in 



the visual field appear to move towards the 

 center of the field, and upon turning the eyes 

 upon an object in the car everything seems to 

 move away from the center of the visual field ; 

 if the train comes to a quick stop while the 

 eyes gaze steadily out at a window the motion 

 of the landscape and the inferred motion 

 of the train appears to be momentarily reversed 

 at stopping, etc. 



The existence of this sensation of motion of 

 stationary objects seems to indicate that neither 

 the succession of stimuli nor the stimulation of 

 successive nerve elements is the fundamental 

 fact in the sensation of motion, but rather 

 that the sensation of motion, like other 

 specific sensations, depends upon a state of 

 nervous commotion, a state which has, of course, 

 resulted from and is the integral effect of a suc- 

 cession of stimuli. A concrete notion of the 

 character of this state of nervous commotion is 

 as follows : 



End organs. 

 A 



_ _ 'V- 

 Cx X X X X X 



Cells of central organ 



X D 



Let the dots A B represent the end organs of 

 sight — rods and cones — and the crosses C D the 

 nerve cells of the central organ. We may im- 

 agine each end organ to be connected, either 

 directly or through ganglion cells, with a num- 

 ber of the cells of the central organ. Let us 

 consider the connections indicated by the diag- 

 onal full lines and dotted lines. A succession 

 of stimuli of the end organs from A to B and 

 a succession from B to A would result in radi- 

 cally different states of nervous commotion, 

 especially if the cross connections are not en- 

 tirely symmetrical or if the connecting nerve 

 fibers are loaded with ganglion cells. Also 

 during a succession of stimuli from A to B the 

 fibers represented by the full lines might be fa- 

 tigued, while the ones indicated by the dotted 

 lines might be saved by inhibition due to the 

 (outgoing) commotion to which they are sub- 

 jected in advance of the moving stimulus, so 

 that the effects of this moving stimulus reach 



