454 JAMES CLERK MAXWELL. [CHAP. XIV. 



first to one minimum visibile, and then to another, so that 

 what is presented to me is like the idea which a blind man 

 forms of the shape of an object by stroking it with the end 

 of his stick. The evidence relates only to the position of 

 points in the line traced by the end of his stick, but he fills 

 in the rest of the surface in accordance with his notions of 

 continuity and probability. 



There is no department of psychophysik which has been 

 so successfully studied as that which relates to vision. When 

 we keep not only our eyes, but our attention, fixed on one 

 small object, the field of conscious vision seems to contract, till 

 only that object remains visible, and even it seems about to 

 disappear. It generally happens, however, that the feeling of 

 uneasiness which grows upon us causes at last a slight dis- 

 placement of the eye, when suddenly a large extent of the 

 field starts into visibility, and the edges of objects, especially 

 those normal to the line of displacement, become obtrusively 

 prominent. This experiment seems at first sight to indicate 

 that the central spot of the retina has some exclusive privi- 

 lege in the economy of vision. What it really shows is that 

 changes in the mode of excitation are essential to perfect 

 vision, and that vision cannot be maintained under an abso- 

 lute sameness of excitation. On the other hand, by means 

 of the instantaneous light of a single electric spark, we may 

 read a whole sentence of print. Here we know that though 

 the illumination lasts only for a few millionths of a second, 

 the image on the retina lasts for a time amply sufficient for 

 an expert reader to go over it letter by letter, and even to 

 detect misprints. This experiment suggests certain specula- 

 tions about memory, a faculty which is often supposed to 

 be essential to the continuity of the Ego in time. 



When men wish to have things remembered, they set up 

 monuments, and write inscriptions and books, they draw 

 pictures and take photographs, in order that these material 

 things may help them, in time to come, to call up the 

 thought of that which they were intended to commemorate. 



In our own bodies we have records of past events. Old 



