WAYSIDE OPTICS. 619 



by indirect vision. But, in looking from my car window, I am 

 made the subject of optical illusions common in a journey of 

 this sort. 



Notwithstanding the many wonderful things about the mech- 

 anism of vision, it exhibits, after all, a great many crudities. In- 

 tellectually, for instance, the optic centers are low down in the 

 scale of origin. Even the olfactory nerves have a higher cerebral 

 origin than they. Accordingly, we often find them committing 

 all sorts of errors, from whose consequences only the experience 

 of the other organs (acting as special detectives) enables the organ- 

 ism to escape. 



Simple " seeing " ought not to be followed, in all cases, by im- 

 plicit belief. When, for example, as in this case, the eye forms 

 part of a moving mass, the motion is wrongly attributed by the 

 optic centers to surrounding bodies. The explanation of how this 

 comes about is easy when one considers certain facts in element- 

 ary optics. If I close one eye and slowly move a pen from right 



Fig. 2. 



• 



to left a few inches in front of the other eye, the direction of the 

 movement is rightly interpreted by my brain, although by a ref- 

 erence to Fig. 2 it will readily be seen that the retinal image of 

 the pen moves in an opposite direction over the background of 

 the eye. 



Precisely the same effect is obtained if, instead of moving the 

 pen, I look straight forward and move my head from left to right, 

 simply because the same impression is produced — i. e., the retinal 

 image moves from left to right. 



When, therefore, the image of an object is made, it matters 

 not hoio, to move over the retinal background, motion in an op- 

 posite direction is immediately referred to the object itself. It 

 makes no difference, then, so far as the optical effect is concerned, 

 whether the solid plain with the objects on its surface be carried 

 past the observer at rest, or whether the observer himself move 

 past or over the plain. Further, when there is no movement of 

 the image over the retina, no motion is detected by the eye ; opti- 

 cally, the object is at a standstill. That a body moving in front 

 of the eye should appear to be stationary, its image must always 

 be kept in the same position on the retina. This is accomplished 



