64 Analyses x>f Books. [Jan. 



although the pupils be actually raised, as it is their condition to 

 be when the eyes are shut, because there is here no sense of 

 voluntary exertion. If we sit at some distance from a lamp 

 which has a cover of ground glass, and fix the eye on the centre 

 of it, and then shut the eye and contemplate the phantom in the 

 eye ; and if, while the image continues to be present of a tine 

 blue colour, we press the eye aside with the finger, we shall not 

 move that phantom or image, although the circle of light pro- 

 duced by the pressure of the finger against the eye-ball moves 

 with the motion of the finger. 



" May not this be accounted for in this manner : the motion 

 produced in the eye-ball not being performed by the appropriate 

 'organs, the voluntary muscles, it conveys no sensation of change 

 to the sensorium, and is not associated with the impression on 

 the retina, so as to affect the idea excited in the mind ? It is 

 owing to the same cause that, when looking on the lamp, by 

 pressing one eye, we can make two images, and we can make the 

 one move over the other. But, if we have received the impression 

 on the retina so as to leave the phantom visible when the eye-lids 

 are shut, we cannot, by pressing one eye, produce any such 

 effect. We cannot, by any degree of pressure, make that image 

 appear to move, but the instant that the eye moves by its volun- 

 tary muscles, the image changes its place ; that is, we produce 

 the two sensations necessary to raise this idea in the mind ; we 

 have the sensation on the retina combined with the conscious- 

 ness or sensation of muscular activity. 



" These experiments and this explanation of the effect of the 

 associated action of the voluntary muscles of the eye-ball, appear 

 to me to remove an obscurity in which this subject has been left 

 by the latest writers. In a most scientific account of the eye 

 and of optics, lately published, it is said on this question, ' we 

 know nothing more than that the mind residing, as it were, in 

 every point of the retina, refers the impression made upon it, at 

 each point, to a direction coinciding with the last portion of the 

 ray which conveys the impression.' The same author says, 

 ' Kepler justly ascribed erect vision from an inverted image to 

 an operation of the mind, by which it traces the rays back to the 

 pupil, and thus refers the lower part of the image to the upper 

 side of the eye.' What can be here meant by the mind follow- 

 ing back the ray through the humours of the eye ? It might as 

 well follow the ray out of the eye, and, like the spider, feel along 

 the line. A much greater authority says we puzzle ourselves 

 without necessity. ' We call that the lower end of an object 

 which is next the ground.' No one can doubt that the obscu- 

 rity here is because the author has not given himself room to 

 illustrate the subject by his known ingenuity and profoundness. 

 But it appears to me, that the utmost ingenuity will be at a loss 

 to devise an explanation of that power by which the eye becomes 

 acquainted with the position and relation of objects, if the seuse 



