ACCESSORY APPARATUS. 399 



higher. It remains now to add that this is only one manner of looking 

 at the thing. According as our hypothesis of the nature of light, of its 

 relations to heat, and of its manner of establishing chemical changes may 

 be, the special explanations we give of the functions of the eye will differ ; 

 yet there is such a relationship among these hypotheses that Translation of 

 we can, without any difficulty, convert an explanation derived the calorific 



n . i i * i / -i T hypothesis to 



irom one into an explanation derived from another. It re- other forms of 

 ally comes to little more than a translation of phraseology. ex P ression - 

 I have found the calorific hypothesis convenient, because we are led to it 

 by the comparative anatomy of the eye in starting from the ocelli of the 

 lower forms ; yet, with almost equal convenience, the function might have 

 been treated otherwise, viewing light as arising from ethereal undulations, 

 the additional advantage then being obtained of establishing a parallel- 

 ism between the action of the organ of sight and that of hearing. Or, 

 in like manner, the case might have been viewed in its purely chemical 

 aspect, photographically, as it might be said, the destruction of the vesic- 

 ular structure of the retina through the agency of arterial oxygen being 

 taken as the primary physical act. But this, again, amounts only to a 

 different mode of stating the same effect, since, as I have shown (London 

 and Edinburgh Philosophical Magazine, May, 1851), all chemical changes 

 accomplished in material substances are occasioned by the establishment 

 of vibratory motions therein, and Ampere has already demonstrated that 

 all the phenomena of heat may be explained upon the doctrine of the vi- 

 brations of the constituent molecules of bodies. 



Divesting ourselves, therefore, of any farther concern in making a se- 

 lection among the various hypotheses, we have adopted the view that the 

 change of the retina originates in a calorific disturbance, because it ap- 

 pears to be somewhat more convenient for our use. 



It is to be understood that the sensation of light is, however, purely 

 mental, and whatever can disturb the nutrition or waste of The sensation 

 the retina will give rise to luminous impressions. The press- of light purely 

 ure of the finger on the ball of the eye, a blow, the passage n 

 of an electric current, and divers other causes, will at once produce the 

 appearance of light, and even of colors. Heat is only one out of a mul- 

 titude of agents that can disturb the retina. 



3d. Of the Accessory Apparatus of the Eye. 



The accessory apparatus of the eye consists chiefly of the eyebrows, 

 the eyelids, the Meibomian glands, the lachrymal mechanism, and the 

 muscles for the movement of the ball. 



The eyebrows are two arches of integument, covered with hair, on the 

 upper edge of the orbit. They are usually classed with the The eyebrows 

 appendages of the eye upon the supposition that they protect and eyelids. 



