240 ANNUAL OF SCIENTIFIC DISCOVERY. 



globe of the eye will be necessary, the central image will change 

 its size, becoming smaller, showing that the reflecting surface has 

 become more convex, and at the same time will change its place 

 to one side, showing that the front of the lens has moved forward. 

 The first and third images undergo little or no change. It is the 

 loss of this power of changing the form of the lens, a power neces- 

 sary to the distinct vision of near objects, that chiefly gives rise to 

 long-sightedness in persons growing old. The inability to accom- 

 modate, according to Bonders, depends upon the lens becoming 

 harder, and therefore less compressive, and so offering greater re- 

 sistance to the ciliary muscle, the chief agent in producing the 

 compression required. When directed to distant objects, the ac- 

 commodating power is at rest, so that the sense of effort is wholly 

 absent. . Most persons are, however, conscious of a distinct effort, 

 and those who are becoming long-sighted, painfully so, when the 

 eye is directed to a near object. It is commonly believed that 

 near-sighted persons, as they grow old, acquire the power of see- 

 ing objects at ordinary distances, because their too convex refract- 

 ing media become flattened with advancing age. This may and 

 does happen to a slight degree in a few, but not in the majority of 

 cases. For the most part, near-sighted persons, as they grow old, 

 find that the near point of distinct vision recedes, while the far 

 point undergoes but little change. This is an important fact in 

 opposition to the theory of flattening, heretofore so generally ac- 

 cepted, and is fully explained by the loss of power to accommo- 

 date. The Nation. 



PERSISTENCE OF THE DIFFERENT COLORS ON THE RETINA. 



Light has been hitherto considered as divisible only into the va- 

 rious colored rays by single, and into polarized rays by double 

 refraction. A new distinction has been discovered, founded on the 

 varied persistence of the impressions made by the different rays 

 on the retina. 



It'has long been known that the impression made by light does 

 not cease with the cause that produces it ; and it has been found 

 that luminous impressions repeated at intervals of time appear to 

 the e}'e continuous. It is on account of such apparent continuity 

 that a stick lighted at one end and made to revolve rapidly round 

 the other as a centre seems to describe a circle of fire. The ap- 

 parent continuity of sensations which are in reality intermitting is 

 not confined to those connected with vision; sounds repeated at 

 very short intervals appear to be uninterrupted. In fact, every 

 sound, however sharp, is but a series of different vibrations. 



The consideration of these facts leads to practical conclusions. 

 If, in ornamentation and music, the sensation of a second color, 

 or sound, may be produced before that of the first has disappeared, 

 the coexistence of colors or sounds, which are primarily intended 

 to act only in succession, must be kept in view ; and the sound or 

 the colors must be such as to produce in the one case musical, and 

 in the other pictorial harmony. 



M. Laborde lias lately communicated to the Academy of Sci- 



