150 ANNUAL OF SCIENTIFIC DISCOVERY. 



eased, the far-sighted and the near-sighted. The former occurs in 

 good eyes, as persons advance in life, beginning about the age of 

 forty, and is remedied by plane, or, better, by convex spectacles. The 

 latter is found in youth, or young persons, and finds its remedy in con- 

 rave glasses. The far-sighted are unable to see near and small ob- 

 jects, and remove them to an inconvenient distance, while they see 

 remoter objects perfectly well without glasses. The near-sighted are 

 unable to see small objects unless they are brought inconveniently 

 near, and they have no distinct vision of remote objects. There is a 

 kind of abnormal vision, different from either of these, which is not 

 far-sighted nor near-sighted, but in which near small objects, or larger 

 distant objects, are not seen with distinctness. This imperfection oc- , 

 cur.s in children and young persons, and is remedied by convex spec- 

 tacles which are suited to the eyes of persons from sixty-five to sev- 

 enty years of age. The younger eyes require the older glasses, and 

 with advanced years less convex glasses are required. At the age of 

 forty-five or more, this kind of abnormal vision becomes much dimin- 

 ished. As the young use the glasses of the far-sighted, this kind 

 may be called neo-macropia. It is evident that convex glasses produce 

 that change in the rays of light which fits such eyes to see distinctly 

 small and large objects at varying distances. This fact proves that 

 there is no defect in the adjusting power of the eyes. The cause, then, 

 is to be sought in the structure of the eye. As this kind of eyes does 

 not appear to be too much or too little convex, and as the image is not 

 formed soon enough in the eye, or is too far back, either or all of the 

 three following may be the cause: 1st, too little convexiiy of the 

 crystalline lens ; or, 2d, its position too near the retina ; or, 3d, its too 

 little density. The second is the probable cause. Spectacles suf- 

 ficiently convex would bring the rays to a focus, let either or all of 

 the three causes operate, and with the usual adjusting power of the 

 eye give distinct vision for near or remote objects. Though this 

 kind of abnormal vision seems not to have attracted attention, for I 

 have found but one allusion to it in consulting authors on optics, it is 

 relatively common. In New England and New York, more than 

 fifty instances of it have come to my knowledge in the five or six 

 years past. A child of fifteen was able to see distinctly, for the first 

 time, by the use of his grandfather's spectacles. A young man of 

 eighteen required convex glasses of ten-inch focus, while persons of 

 seventy years use those of fourteen to eighteen inch focus. Children 

 often make little progress in study, because they do not see objects 

 distinctly, though the defect is not suspected by them, and is utterly 

 unknown to parents and teachers. The knowledge of this subject 

 will make spectacles a still greater benefit to our race. 



PHYSIOLOGICAL ACOUSTICS. 



IN order to ascertain the causes of the same body communicating to 

 our ear different tones at the same time, M. Duhamel has made the 

 following experiment. A caoutchouc thread connected consecutively 

 with different points of an oscillating plate, producing simultaneously 



