20 ADAPTATION OF THE EYE TO VISION 



numerous oil-like globules, which are probably the nuclei of the disinte- 

 grated cells. 



Between the above-described cells immediately surrounding the fibrous 

 portion of the retina, and the internal surface of this membrane, other 

 globular particles are found, smaller than the last.* They somewhat 

 resemble blood- corpuscles, in form and size, and lie thickly together, but 

 without adhering either to each other, or to the larger corpuscles. As 

 was before observed, they are considered by Henle to be an early stage 

 of development of the large cells, probably the nuclei; many, indeed, 

 appear to be already surrounded with a delicate cell-wall, within which 

 is a faintly-granular material. These cells occasionally present a very 

 close resemblance to epithelium, f 



Formation of images on the retina. It has been found by Volkmann J 

 that in order to perceive the image of a bright object depicted on the 

 retina of a human eye, it is not necessary to make an opening into the 

 sclerotic and choroid coats, as formerly directed, for it can be perceived 

 through these tunics almost as distinctly as through the transparent 

 tissues of the eye of the white rabbit or other albino animal. More- 

 over, he has found that this image may be observed in the eye even 

 of a living person. For this purpose an individual should be selected 

 in whom the eyes are large and prominent, and whose sclerotica pos- 

 sesses an unusual degree of transparency, as denoted by the bluish tint 

 which it presents through the conjunctiva. When such an eye is directed 

 as far outwards as possible, and a luminous object is then placed at the 

 outside of it, at an angle of from 80 to 85, the image of this object may 

 be detected at the inner angle of the eye, appearing through the trans- 

 parent sclerotica. Sometimes this image is so distinct that the inverted 

 position in which the object is depicted on the retina may be clearly 

 discerned. 



Adaptation of the eye to vision at different distances.^ 



The power possessed by the eye of so adapting itself as to obtain a dis- 

 tinct view of objects placed at various distances from it, and thus of pro- 

 viding against the errors in vision which would otherwise result from the 

 varying focal distances at which the perfect image of the objects would be 

 formed on the retina, still continues to occupy the attention of physiolo- 

 gists ; and although many ingenious attempts, both formerly and of late, 

 have been made to account for this highly important property of the eye, 

 no completely satisfactory explanation of it has been hitherto afforded. 



* Valentin, Henle, &c. Op. cit. 



f Klenke, in Canstatt's Jahresbericht for 1842, Physiologic, s. 315. 



J Wagner's Handwbrterbuch, Art. Sehen, p. 286. 



$ Miiller's Physiology, p. 1131. || Miiller's Physiology, p. 1136. 



