RETINAL STIMULATION OPTOGRAM. 



773 



interrupted line that for red 

 of colours are as under : 



is the blind spot. In the normal eye the limits for the perception 



Externally, 

 Internally, . 

 Upwards, , 

 Downwards, 



V. Specific Energy. The rods and cones alone are endowed with what Johannes 

 Muller called "specific energy" i.e., they alone are set into activity by the ethereal 

 vibrations, to produce those impulses which result in vision. Mechanical and 

 electrical stimuli, however, when applied to any part of the course of the nervous 

 apparatus, produce visual phenomena. Mechanical stimuli are more intense stimuli 

 than light rays, as is shown by performing the dark pressure figure with the eyes 

 open ( 393, 5, a), whereby the circulation in the retina is interfered with ; in the 

 region of pressure, we cannot see external objects which affect the retina uniformly 

 and continuously. 



VI. The duration of the retinal stimulation must be exceedingly short, as the 

 electrical spark lasts only - 000000868 second ; still, as a general rule, a shorter 

 time is required, the larger and brighter the object looked at. Alternate stimu- 

 lation with light, 17 to 18 times per minute, is perceived most intensely (Briicke). 

 Further, an increase or diminution of 0'01 part of the intensity of the light is 

 perceptible ( 383). A shorter time is required to perceive yellow than is required 

 for violet and red ( Vierwdt). The retina becomes more sensitive to light, after a 

 person has been kept in the dark for a long time, and also after repose during the 

 night. If light be allowed to act on the eyes for a long time, and especially if it 

 be intense, it causes fatigue of the retina, which begins sooner in the centre than 

 in the periphery of the organ (Aubert). At first the fatigue comes on rapidly and 

 afterwards develops more slowly it is most marked in the morning (A. Fide). 

 The periphery of the retina is specially characterised by its capacity for distinguish- 

 ing movements (Exner). 



VII. Visual Purple. The mode of the action of light upon the end-organs of 

 the retina has already been referred to (p. 739) in connection with the "visual purple" 

 or rhodopsin {Boll, Kiihne). Kiihne showed that, by illuminating the retina, actual 

 pictures (e.g., the image of a window) could be produced on the retina, but they 

 gradually disappeared. From this point of view we might regard the retina as 

 comparable, to a certain extent, to the sensitive plate of a photographic apparatus. 



Optogram. The visual purple is formed by the pigment-epithelium of the retina. Perhaps 

 we might compare the process to a kind of secretion. The visual purple may be restored in a 

 retina by laying the latter upon living choroidal epithelium. The pigment disappears from 

 the mammalian retina by the action of light 60 times more rapidly than from the retina of the 

 frog. In a rabbit's eye, whose pupil was dilated with atropin, Ewald and Kiihne obtained a 

 sharp picture or optogram of a bright object placed at a distance of 24 cm. from the eye the 

 image was "fixed by a 4 per cent, solution of alum. Visual purple withstands all the oxidising 

 reagents ; zinc chloride, acetic acid, and corrosive sublimate change it into a yellow substance 

 it becomes white only through the action of light ; the dark heat-rays are without effect, 

 while it is decomposed above a temperature of 52 C. [As visual purple is absent from the 

 cones, and cones only are present in the fovea centralis, we cannot explain vision by optograms 

 formed by the visual purple.] 



VIII. Destruction of the rods and cones of the retina causes corresponding 

 dark spots in the field of vision. 



396. PERCEPTION OF COLOURS. Physical. The vibrations of the light-ether are per- 

 ceived by the retina only within distinct limits. If a beam of white light, e.g., from the sun, 

 be transmitted through a prism, the light rays are refracted and dispersed, and a "prismatic 

 spectrum" is obtained (fig. 17). White light contains rays of very different wave-lengths or 

 periods of vibration. The dark heat-rays, whose wave-length is 0*00194"mm., are refracted least 

 (Fizeau). They do not act upon the retina, and are therefore invisible. They act, however, 



