CHAP. XVII.] DURATION OF IMPRESSIONS OCULAR SPECTRA. 53 



under all degrees of illumination. The excitability of the retina in 

 creatures usually exposed to the full light of day, requires this 

 additional protection; and where it is deficient, as in albinos, an 

 ordinary light becomes painful, and the moveable protecting parts 

 are habitually brought into increased use. In animals of nocturnal 

 habits, furnished with a tapetum lucidum, the excitability of the 

 retina is probably somewhat modified, and the iris also is generally 

 larger, and capable of an ampler range of motion. 



Duration of Impressions on the Retina. We continue to see an 

 object after the rays of light emerging from it have ceased to fall 

 upon the retina, and this for a period proportioned to the intensity 

 and duration of the impression they have left. The familiar ex- 

 periment of twirling a lighted stick, so as to see a luminous circle, 

 shows that the impression made by it, when at any one point of 

 space, remains on the retina until it reaches that point again. By 

 ascertaining the speed necessary for completing a luminous circle of 

 a certain size we can estimate the duration of the impression; and 

 by augmenting or diminishing the brilliancy of the ignited point its 

 duration is found to be affected. A momentary impression of 

 moderate intensity continues for a fraction (according to D'Arcy, 

 about an eighth part) of a second. But if the impression be 

 made for a considerable time on any one point of the retina, it 

 endures for a longer period after the object is removed. It is 

 owing to this retentive power of the retina, that the rapid and in- 

 voluntary act of winking does not interfere with continuous vision 

 of surrounding objects. 



Appearances of objects remaining after the removal of the objects 

 themselves from the field of vision, come under the head of ocular 

 spectra. In figure they correspond to the image the object has 

 thrown on the retina, but they are of the complementary colour 

 to that of the object. Thus, the spectrum left by a red spot is 

 green; by a violet spot, yellow; by a blue spot, orange; and these 

 colours of the spectra are particularly obvious when the eye is di- 

 rected towards a white ground. It is further remarkable, that after 

 long gazing on a very bright light, as the sun's disc, the remaining 

 spectrum, if viewed on a white surface, assumes the different co- 

 lours in succession, from black, through blue, green, and yellow, 

 to white : if viewed on a black surface, the order of the succession 

 is reversed. These several phenomena can only be referred to par- 

 ticular states or modes of excitation of the retina ; by means of which 

 alone it is that the differences of the component colours of white 

 light are made evident to our perceptive powers. 



