VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 345 



of the crystalline humour being muscular, and proposed to make it the subject of 

 the Croonian lecture, I did not foresee that any thing could prevent me from 

 fulfilling my promise; but since that time, what with my state of health, which 

 does not allow me to be very active; the hurry of official business on account of 

 lithe war, and my brother-in-law, Mr. Home, being employed on the medical 

 staff, I have not had the power of repeating my experiments, and drawing out, 

 to my satisfaction, the many conclusions which are the result of such a power in 

 this humour. 



The laws of optics are so well understood, and the knowledge of the eye, 

 when considered as an optical instrument, has been rendered so perfect, that I 

 do not consider myself capable of making any addition to it; but still there is a 

 power in the eye by which it can adapt itself to different distances far too ex- 

 tensive for the simple mechanism of the parts to effect. This power, writers on 

 this subject have been at great pains to investigate and explain. The motion of 

 the crystalline humour forwards and backwards, was asserted by some to be the 

 cause; while others supposed in the eye a power to alter its shape, so as to shorten 

 or lengthen its axis, which altered the distance between the crystalline humour 

 and the point of impression ; but we should consider that a part of the eye is 

 itself a refractor, and that if its shape be altered so as to remove the crystalline 

 humour from the point of impression, in order to enable it to bring a distant 

 object to its proper focus on the retina, this effect will be in some degree 

 counteracted by the anterior part of the eye refracting more than before, by 

 being rendered more convex. But we have, in fact, no power capable of pro- 

 ducing this effect; for the straight muscles, so far from appearing to have this 

 power, have been even supposed to flatten the eye, and shorten its axis: and it 

 is very possible that the action of these muscles is such as tends to both effects; 

 but being in opposition to each other, the eye retains its shape, the insertion of 

 these muscles being much more forwards than appears to be necessary for the 

 simple motions of the eye. Further, when we consider that in many animals 

 the shape of the eye is unalterable, as in all of the whale tribe, the sclerotic 

 coat being above half an inch thick, and composed of a strong tendinous sub- 

 stance. In many fish this coat is composed of cartilage; and in all birds the an- 

 terior part of it is I believe composed of bone. From all these considerations, I 

 saw no power that could adapt the eye to the various distances of which we find 

 it capable in the human body, unless we suppose the crystalline humour to be 

 varied in figure, which can only be effected by a muscular action within itself. 

 With this idea strongly impressed upon my mind, and finding that in many 

 animals, when the crystalline humour was coagulated, it had a fibrous structure 

 like muscles, I confess it seemed to me to confirm it; but as this might to 

 others appear only conjecture, requiring some proof, I set about such experi- 



voL. XVII. y Y 



