VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 407 



finest hairs of equal thickness, arranged in similar order; see fig. 4, 5, and 6, 

 where the arrangement of the layers and fibres has been painted from the real 

 lens of an ox, and that without the assistance of a glass. To observe this fact, 

 any person may try the experiment at pleasure, and witness the same with the 

 naked eye, even separating many layers and their fibres with the point of a pen- 

 knife. This regular structure of layers, and those consisting of concentric fibres, 

 is unquestionably better adapted for the transmission of the rays of light, than 

 the irregular structure of muscles. It may perhaps be urged, that the heat to 

 which I exposed the lens may have changed its structure: in answer to that I 

 observe, it was moderate in degree, and regularly applied; of consequence we 

 may presume, as it appeared uniformly opaque, that every part was alike acted 

 on; but by boiling the lens, where the heat is, without doubt, regularly applied, 

 we observe the same structure. 



3dly, that it is not from any changes of the lens, and that this is not the most 

 essential organ in viewing objects at different distances, we may also infer from 

 this undeniable fact, that we can in a great degree do without it; as after couch- 

 ing or extraction, by which operations all its parts must be destroyed, capsule, 

 ciliary processes, muscles, &c. Mr. Young asserts, from the authority of Dr.' 

 Porterfield, that patients, after the operation of couching, have not the power 

 of accommodating the eye to the different distances of objects; at present I 

 believe the contrary fact is almost universally asserted*. 



Besides, if the other powers of the eye are insufficient to compensate for the 

 loss of this dense medium, the lens, a glass of the same shape answers the pur- 

 pose, and which certainly does not act by changing its figure. I grant their 

 vision is not so perfect; but we have other circumstances on which this can be 

 more easily explained ; which will be particularly noticed under the next head. 

 It may not be improper also to observe, that the specific gravity of the crystalline 

 compared with that of the vitreous humour, and of consequence its density and 

 power of refraction, is not so great as has been generally believed. Dr. Bryant 

 Robinson, by the hydrostatic balance, found it to be nearly as 11 to 10. I have 

 also examined them with the instrument of Mr. Schmeisser, lately presented to 

 tlie R. s., and found the same result; of consequence the crystalline lens is not 

 so essentially necessary for vision as has been represented; especially as it is also 



* "■ Et lente ob cataractam extractam vel depositam oculum tamen ad varias distantias videre, ut in 

 nobili viro video absque ullo experimento qui earn facullatem recuperaverit. Etsi enini tunc ob di- 

 minutas vires quae radios uniunt, aeger lente vitrea opus habet, eadem tamen lens in omni distantia 

 sufficit."— Haller, El. Phys. 



" La lentille cnstalline n'est cependant point de premiire necessitc pour la vision. Aujourd'hui, 

 dans rop> ration de la cataracte on I'enlcve entierement, et la vision n'en souffre point," — De la Me- 

 therie Vues Physiologiques. See also De la Hire, Hamberger Pb;^siolog. — Orig. 



