578 SIGHT. 



which assumes that any colour presented to the retina stimulates 

 it and excites it to a reverse action which produces the comple- 

 mentary colour. Professor Plateau has supported this theory 

 with much ingenuity, but there are numerous facts which do not 

 accord with it. 



Longsightedness and shortsightedness. That case of defective 

 vision which is called longsightedness arises from the crystalline 

 lens being too flat; the rays proceeding from near objects, instead 

 of converging to distinct foci on the retina, converge behind it, 

 and therefore form no distinct image. This defect, which is an 

 ordinary effect of old age, may be remedied by the use of a 

 convex lens, which enables the eye to converge the rays so as to 

 form a perfect image on the retina. 11 



Shortsighted persons are unable to see at a distance, and are 

 obliged to bring small objects very near the eye to see them 

 distinctly. This defect, which often occurs in young persons, 

 arises from the eye being too convex, from which cause the rays 

 of distant objects converge to foci before they fall on the retina. 

 The imperfection may be remedied by using a concave lens, 

 which renders the rays less converging and enables them to form 

 a distinct picture at the bottom of the eye. There are other 

 cases of defective vision arising from the malformation of the 

 organ, but these are of the most common occurrence. 



n In hemiplegia, a sense sometimes becomes morbidly acute. Dr. Heberden 

 (Comment, p. 292.) mentions a hemiplegic person whose smell became greatly 

 heightened. Frequently we find such patients sensible to the crawling of the 

 minutest insect on the arm. I lately attended a gentleman about forty years of 

 age, who had suddenly been attacked with hemiplegia, and in bed he heard the 

 least sound at the bottom of the house with an acuteness which surprised him, 

 and could tell the hour by a watch placed on a table at such a distance from his 

 bed as to have rendered it impossible for him to distinguish the hands when he was 

 in health. Dr. Brachet relates that, when he was interne at the Bicetre in 1811, 

 the infirmier of the surgical ward one day astonished him by the extent which 

 his vision had acquired since the day before. The man could distinguish the 

 most minute objects at an enormous distance. Five hours afterwards he felt a 

 slight headach, and in a few hours more was seized with a thundering apoplexy 

 (une apoplexie foudroyante), and died the next night. A fresh coagulum was 

 found in the right optic thalamus. The inflammation which had preceded this 

 effusion had irritated by its proximity a part of the brain concerned in vision. 

 These were instances of longsightedness ; but not of mere longsightedness, but 

 general acuteness of sight, as the persons saw well not only at great distances, 

 but at small distances likewise. 



