Description of an Organ 



[March, 



Article 11 L 



The Description of an Organ ly ivhich the Eyes of Birds are 

 accommodated to the different Distances of Objects* By Philip 

 Crampton, Esq. 



(With a Plate.) 



In the philosophical investigation of the phenomena of vision 

 tlse eye must be considered under two aspects — as an optical 

 instrument, and as an organ of sense. This mixed nature of 

 the organ subjects the analysis of its functions, upon mathemati- 

 cal principles, to considerable uncertainty ; for the perceptions of 

 siglit muse be considered as depending upon certain motions or 

 affections of the nervous power, which, although originally 

 excited by the agency of light, may exist or be renewed without 

 its presence. In deliriums, for example, the perception of 

 objects that have no existence is as perfect as if their images 

 were formed by rays accurately converging upon the retina. In 

 addition to this, it may be observed, tliat the eye is not a 

 perfectly achromatic instrument, yet ol)iects are not seen with 

 that kind of indistinctness which might be supposed to result 

 from this imperfection in its optical constitution. It appears, then, 

 tliat in the present state of our knowledge it must be difficult, if 

 not impossible, to ascertain how much, if any, apparently 

 optical effect is to be attributed to the mechanical constitution of 

 the eye, and how much to the agency of the principle of life. 

 This cannot be better illustrated than by referring to the contra- 

 dictory opinions which have been maintained among the most 

 4!istingui^hed natural philosophers, with respect to the faculty 

 which the eye is thought to possess, of adjusting its focus to the 

 diiterent distances of objects. All the hypotheses which have 

 been framed, to explain the means by which this adjustment is 

 f&cted, have proceeded upon the su})position that it was neces- 

 sarily connected with some change, either in the external conli- 

 -garation of the eye, or in the relative position of its internal 

 parts. Now, although it is certain that to form a perfectly 

 distinct image u|70d the retina, the focus of the eye must be 

 ^accommodated to the distance of the object, still we have as yet 

 BO proof that such a perjt'ct image is essential to distinct vision. 



The nervous ioHuence, whatever may be its nature, which 

 conveys to the seosorium a knowledge of those properties of 

 bodies v/hich are the proper objects of vision, may be as perfectly 

 excited by rays possessing one degree of convergency as another; 

 for we cannot entertain the gross conception of the mind sitting 

 belnnd the eye to contemplate the pictures which are painted 

 upon the retina. The impressions of light and colours upon the 

 eye, like tb,e impressions of articulate sounds upon the car. 



