134- Dr. Wollaston on Semi-decussation 



ducting hydrate of potassa, suffers a diminution of capacity 

 for heat, while the oxygen gas gains an increase : thus the 

 potassium has an excess of radiant matter to impart, while at 

 the negative extremity of the pile there is a deficiency; there- 

 fore the potassium and copper naturally attract each other. 

 The reason why oxygen should be attracted by that pole where 

 radiant matter is in excess, and thereby have a tendency im- 

 parted to it of combining with metallic matter, may readily be 

 admitted. Why in common cases do we heat a metal in order 

 to combine it with oxygen ? Is it to furnish the latent heat 

 which the increase of capacity of the newly formed oxide de- 

 mands ? Yes. 

 East Smithfield, July. 28, 1824. THOMAS POLLOCK. 



XXII. Of Semi-decussation of the Optic Nerves. By Willi am 



Hyde Wollaston, M.D. V.P.R.S* 

 "ITTHETHER we consider the astonishing subtlety of that 

 * * medium which renders visible to us objects existing 

 at the most immeasurable distances from us, or that delicately 

 constituted organ which, by its general structure, collects the 

 rays of light, and by a nice adaptation of its parts concen- 

 trates their force on the sentient fibres of the retina, expanded 

 over its inner surface, we can feel no surprise that such great 

 talents should have been devoted to investigate the curious 

 properties of the one, or that the structure of the other should 

 have been examined with so much assiduity. 



The keenness of inquiry manifested by the cultivators of 

 anatomy, in observing the most minute parts that have escaped 

 the notice of their predecessors, shows that any addition to 

 the common stock of our information on this subject will be 

 gratifying to a certain portion of the members of this Society, 

 and probably not uninteresting to the Society at large. 



It is not my object, in the present paper, to examine either 

 the first effect of the cornea in rendering the rays of light 

 convergent, or the power of the crystalline lens in finally 

 bringing them to a focus on the retina. It is not my intention 

 to investigate whether the adaptation of the eye to different 

 distances is effected by alteration of the form of the lens from 

 its own muscular structure, or by alteration of its ■place., from 

 the agency of other muscles. Nor do I mean to consider 

 either the involuntary motions of the iris dependent on the 

 quantity of light present, or that voluntary contraction of it 

 by which we adapt the aperture of the pupil for distinct vision 



* From the Philosophical Transactions for 1824, Part I. 



at 



