2k* INVERSE PHYSIOLOGICAL PROBLEMS. 



over, there must be an adaptation between the nature of that agent and 

 the structure thus brought in relation with it, as is strikingly illustrated 

 by each of the organs of sense. Thus the peripheral extremities of the 

 fibrils of the optic nerve are involved in a combination of a purely phys- 

 ical kind, having relation to the properties of light : the convex surface 

 of the cornea, the unequicurved lens, the diaphragmatic iris, the interior 

 investiture of black pigment, these are all structures the object of which 

 we clearly understand. We know that the rays of light must undergo 

 refraction at the curved surfaces upon which they are incident, and de- 

 pict the images of external forms on the retina or black pigment, the iris 

 expanding or contracting, as the case may be, to regulate the entrance of 

 Adaptation be- the light. So completely do we admit this principle of an 

 !* adaptation of structure to the nature of the agent which is 

 to set it in activity, that in this particular instance, without 

 any hesitation, we class the eye among optical instruments, and include 

 its description in our optical treatises. But in the same manner that, 

 starting from the well-known properties of light, we advance to the ex- 

 planation of the uses of each of the various parts of the eye, there can be 

 no doubt that the converse of this method of reasoning would be possi- 

 ble to an intellect of sufficient power, who, from a full consideration of 

 the structure of the eye, might determine the properties of light, guided 

 in doing this by the principle that there must be an adaptation between 

 such structures and such properties ; and, in the same manner, a man 

 deaf and dumb, but of an intellect of great capacity, might doubtless, 

 from the critical study of the construction of the ear, determine the na- 

 ture of sounds. Nay, even more, it is not impossible that he should be 

 able to compare together the physical peculiarity of the movements which 

 constitute light or sound respectively, and to demonstrate that these 

 originate in normal, and those in transverse vibrations. 



So, therefore, these problems present themselves under a double aspect, 

 Nature of in- and are capable both of a direct and an inverse solution : 

 Mcai h prob- Given * ne nature of light, to determine what must necessarily 

 lems. be the construction of the organ of vision ; or, Given the con- 



struction of the eye, to determine what is the nature of light ; and the 

 same might be said of the organ of hearing. This inverse method of 

 treating natural agents is still in its infancy, because of the extreme im- 

 perfection of our knowledge ; but doubtless what has been said will re- 

 call to the mind of the reader the parallel example which is furnished by 

 astronomy, and which, within a few years past, has yielded such a splen- 

 did result. The mass of a planet being known, the perturbations which 

 it can cause in another are capable of direct computation, but it was re- 

 served for Leverrier to discuss the inverse problem, and from the per- 

 turbations to find the place of the planet. The discovery of Neptune 

 was the result. 



