EFFECTS OF BRIGHT LIGHT UPON THE EYES 47 



unprotected eyes. Every one knows that looking at a bright 

 light is followed by an unpleasant blurred spot in the centre of 

 the field of vision. This spot, which is known as a negative 

 after-image, lasts for a variable period according to the intensity 

 of the light and the length of time during which it is fixed with 

 the eyes. The commonest sequel of observing an eclipse of the 

 sun with unprotected eyes, the so-called eclipse blindness, is 

 a pathological exaggeration of this physiological phenomenon. 

 The after-image, instead of gradually dwindling and finally 

 disappearing, persists as a permanent blind spot in the centre 

 of the field of vision. In technical language this is described 

 as a permanent positive central scotoma. It has an extremely 

 disastrous effect upon vision because it affects the most sensitive 

 part of the sentient layer of the eye or retina. It abolishes 

 vision in that part which is used for the things we are actually 

 looking at. The surrounding retina is still functional but vision 

 is normally far less acute and is useless for the observation of 

 the fine details of things seen. The total effect is that, while 

 the patient is quite able to walk about, everything is seen 

 dimly and he is unable to read or do other near work. If the 

 retina of such a patient be examined by the ophthalmoscope the 

 central area or macula lutea is seen to be diseased. Instead of 

 being red it is white in the early stages but later on shows 

 minute spots of black pigment. As already mentioned, the con- 

 dition is permanent in the majority of cases. Other cases occur 

 in which the damage is still greater. Whether or no the changes 

 mentioned appear, vision gradually fails not only in the central 

 areas but over the whole field of vision. Inspection with the 

 ophthalmoscope then shows that the optic nerve, which conducts 

 the visual impulses from the retina to the brain, is atrophic and 

 dead. Other rarer results of the action of bright sunlight have 

 been noticed but need not detain us. 



Nearly comparable with eclipse blindness is the effect some- 

 times observed after exposure of the eyes to the bright flash 

 which occurs when a powerful electric current becomes short- 

 circuited. Here the duration of the stimulus is momentary 

 but the intensity of the light is extreme. The changes in the 

 eyes which have generally been seen in these cases are almost 

 identical with those following exposure to bright sunlight. 



Allied to these conditions again are the cases of injury to the 

 eyes from lightning stroke. Here, however, we are dealing not 



