ioO LECTURE XXXVIII. 



retina should be of that precise form, which is exactly suited to the reception 

 of the image to be depicted on it. 



The whole surface of the retina appears to be usually occupied by such an 

 image, but it is not all of equal sensibility; a certain portion only, near the 

 axis, is capable of conveying distinct impressions of minute objects. But 

 the perfection of this limited distinctness is a far greater advantage to us, 

 than a more extensive field of moderately accurate vision would have been; 

 for by means of the external muscles, we can easily so change the position 

 of the eye, that the image of any object before us may be maile to fall on 

 the most sensible part of the retina. We may readily observe the want of 

 sensation at the entrance of the optic nerve, by placing two candles so that 

 the distance of each from the eye may be about four times their distance 

 from each other: then if we direct our right eye to to the left hand candle, 

 the right hand candle will be lost in a confused mass of faint light, its image 

 on the retina falling on the point at which its sensibility is deficient. 



When the attention is not directed to any particular object of sight, the 

 refractive powers of the eye are adapted to the formation of an image of 

 objects at a certain distance only, which is different in different individuals, 

 and also generally increases with increasing age. Tims, if we open our 

 eyelids suddenly, without particular preparation, we find that distant objects 

 onlv appear as distinct as we are able to make them ; but by an exertion of 

 the will, the eye may be accommodated to the distinct perception of nearer 

 objects, yet not of objects within certain limits. Between the ages of 40 

 and and 50, the refractive powers of the eye usually begin to diminish, but 

 it sometimes happens that where they are already too great, the defect con- 

 tinues unaltered to an advanced age. It appears also that after 50 or 60, 

 the power of changing the focus of the eye is always much impaired, and 

 sometimes wholly lost. 



The mode, in which the accommodation of the eye to different distances is 

 effected, has long been a subject of investigation and dispute among opti- 

 cians and physiologists, but I apprehend that at present there is little further 

 loom for doubting, that the change is produced by an increase of the con^ 



