APPLICATION OP THE ARGtJ3IENT. !/> 



the known laws of mechanism taking place ; whereas, ii^ 

 the automaton, for the comparatively few motions of which 

 it is capable, we trace the mechanism throughout. But, 

 up to the limit, the reasoning is as clear and certain in the 

 one case as the other. In the example before us, it is a 

 matter of certainty, because it is a matter which experience 

 and observation demonstrate, that the formation of an im- 

 age at the bottom of the eye is necessary to perfect vision. 

 The image itself can be shown. Whatever affects the dis- 

 tinctness of the image, affects the distinctness of the vision. 

 The formation then of^ such an image being necessary, (no 

 matter how,) to the sense of sight, and to the exercise of 

 that sense, the apparatus by which it is formed is con- 

 structed and put together, not only with infinitely more art, 

 but upon the selfsame principles of art, as in the telescope 

 or the camera ol-scura. The perception arising from the 

 inaage may be laid out of the question ; for the production 

 of the image, these are instruments of the same kind. 

 The end is the same; the means are the same. The pur- 

 pose in both is alike , the contrivance for accomplishing 

 that purpose is- in both alike. The lenses of the telescope, 

 and the humours of the eye, bear a complete resemblance 

 to one another, in their figure, their position, and in their 

 power over the rays of light, viz. in bringing each pen- 

 cil to a point at the right distance from the lens ; name- 

 ly, in the eye, at the exact place where the membrane is 

 spread to receive it. How is it possible, under circum- 

 stances of such close affinity, and under the operation of 

 equal evidence, to exclude contrivance from the one, yet 

 to acknowledge tlie proof of contrivance having been em- 

 ployed, as the plainest and clearest of all propositions, in 

 the other ? 



The resemblance between the two cases is still more accu- 

 rate, and obtains in more points than we have yet represented, 

 or than we are, on the first view of the subject, aware of. 

 In dioptric telescopes there is an imperfection of this na- 

 ture. — Pencils of light, in passing through glass lenses, 

 are separated into different colours, thereby tinging the ob- 

 ject, especially edges of it, as if it were viewed through a 

 prism. To correct this inconvenience had been long a 

 desideratum in the art. At last it came into the mind of 

 a sagacious optician, to inquire how this matter was man- 

 aged in the eye ; in which there was exactly the same diffi- 

 culty to contend with as in the telescope. His observation 

 '^iglil him, that, in the eye, the evil was cured by com- 



