836 THE EYE AND VISION [CH. LVI1I. 



the distance of the screen from the eye, the angle through which the 

 candle is moved, and the displacement of the figure seen. 



Duration of Visual Sensations. The duration of the sensation 

 produced by a luminous impression on the retina is always greater 

 than that of the impression which produces it. However brief the 

 luminous impression, the effect on the retina always lasts for about 

 one-eighth of a second. Thus, supposing an object in motion, say a 

 horse, to be revealed on a dark night by a flash of lightning. The 

 object would be seen apparently for an eighth of a second, but it 

 would not appear in motion ; because, although the image remained 

 on the retina for this time, it was really revealed for such an 

 extremely short period (a flash of lightning lasting only a millionth 

 of a second) that no appreciable movement on the part of the object 

 could have taken place in the period during which it was revealed to 

 the retina of the observer. The same fact is proved in a reverse 

 way. The spokes of a rapidly revolving wheel are not seen as 

 distinct objects, because at every point of the field of vision over 

 which the revolving spokes pass, a given impression has not faded 

 before another replaces it. Thus every part of the interior of the 

 wheel appears occupied. 



The stimuli which excite the retina are exceedingly slight ; for instance, the 

 minimum stimulus in the form of green light is equal in terms of work to that which 

 is done in raising a ten-millionth part of a milligramme to the height of a millimetre, 

 and even some of this is doubtless wasted in the form of heat. The time during 

 which the stimulus acts may be excessively small ; thus light from a rapidly rotating 

 mirror is visible even when it only falls upon the retina for one eight-millionth part 

 of a second. Some physiologists have drawn an analogy between retinal and 

 muscular excitations. There is no complete analogy, but the following points of 

 resemblance may be noted : 



1. The retina like the muscle possesses a store of potential energy, which the 

 stimulus serves to fire off. 



2. Fatigue on action, and recovery after rest are noticeable in both. 



3. The curve of retinal excitation, like the muscle curve, rises not abruptly but 

 gradually to its full height, and on the cessation of the stimulus takes a measurable 

 time to fall again, the retinal impression outlasting the stimulus by about one-eighth 

 of a second. 



4. With comparatively slow intermittent excitation, the phenomenon known as 

 flicker takes place ; this may be shown by the slow rotation on Maxwell's machine 

 of a disc painted with alternate black and white sectors. This roughly corresponds 

 with what in a muscle is called incomplete tetanus. 



5. When the rate of stimulation is increased, as by increasing the speed of rota- 

 tion of the disc just alluded to (say to twenty or thirty times a second) the resulting 

 sensation is a smooth one of greyness. This fusion of individual stimuli into a con- 

 tinuous sensation does not by any means correspond to the complete tetanus of 

 muscle, for the resultant sensation has a brightness corresponding not to a summa- 

 tion of the individual fusing sensations, but to a brightness which would ensue if the 

 stimuli were spread evenly over the surface of the disc (Talbot's Law). 



The Ophthalmoscope. 



Every one is perfectly familiar with the fact, that it is quite im- 

 possible to see thefundus or back of another person's eye by simply 



