356 PHYSIOLOGY CHAP. 



which ought to be plain at a distance of 15 m., his visual acuity is expressed 

 in the fraction ^ = . 



The test is carried out as follows : The patient is placed at a distance of 

 5 m. from Snellen's type, and one eye at a time is examined, while the other 

 is cut out by an opaque screen. Occlusion of the eye by the hand or by 

 closing the muscles of the lid should be avoided, because in the subsequent 

 examination this eye would then appear to have less visual acuity than is 

 actually the case. Before commencing the experiment any ametropia must 

 be corrected in the patient by means of concave, convex, or cylindrical lenses. 

 He is then asked to read off the letters of the table beginning with the largest : 

 the smallest he is able to read give the measure of his visual acuity, the 

 arithmetical expression of which is shown by the numerals marked at the 

 end of, or below, each row of letters. 



The excitation of the retina and the sensation evoked are 

 further dependent on the duration of the luminous stimulus. 

 When this is too short, no sensation results unless the intensity 

 of the stimulus is excessive as with a flash or electric spark, 

 which is perceived by the eye even if it lasts for an immeasurably 

 short time. 



Whether retinal activity is preceded by a brief latent period 

 has not been, and perhaps cannot be, demonstrated, but it is known 

 that a certain time (from 0-07 to 0-16 sec.) elapses after the com- 

 mencement of the action of light before the visual sensation 

 reaches its full intensity. For this reason, a bright light acting 

 for a very short time seems less vivid than the same light acting 

 for a longer time. After reaching its maximum by a certain curve 

 called by Fick and Exner the curve of rise or waxing (Anklingen) 

 the sensation remains constant for a longer or shorter period. 

 This is the period best known to us, because it lasts the longest, 

 and our attention is usually given to it. The climax of intensity 

 is usually succeeded by a period of fall or waning (Abklingen), 

 which only appears when we continue to look steadily at the 

 source of light. When the light ceases to act the sensation dis- 

 appears gradually after a certain ' time. This is the period of 

 after-effect, which again declines progressively, and is the cause of 

 positive after-images. 



The phases of visual sensations during the action of light, and 

 their persistence after the stimulus has ceased, can be demonstrated 

 by a very simple experiment with rotating discs. The disc re- 

 presented in Fig. 173 contains the same~number of white sectors, 

 which reflect a great deal of light, and black sectors, which reflect 

 very little light. When it is rotated at a low velocity the white 

 and black sectors are seen distinctly. But on increasing the speed 

 of rotation the edges of the sectors become blurred. This proves 

 that the sensation of black or white does not reach its maximal 

 intensity at once, but only after a certain lapse of time (waxing 

 phase) ; also that at the close of either stimulus the corresponding 

 sensation does not cease suddenly but persists for a certain time 

 (waning phase). 



