310 NATURAL THEOLOGY. 



moderate light the pupil is moderately expanded ; 

 but when the light is obscure the circular fibres 

 relax, and the straight fibres act ; the iris is then 

 diminished in diameter, and the pupil enlarged. 

 The contrary takes place when too intense a light 

 strikes into the eye. 



This guardian action of the iris is more rapid 

 than words, and as quick as thought ; and it is to 

 be remarked that this apparatus is animated by 

 nerves which go back to the sensorium : so that 

 the impression must be received in the sensorium 

 before the iris can be directed in its motions. 



Such, then, is the apparatus by which the nerve 

 of vision is guarded ; and as marking its necessity, 

 let us remember that the retina is susceptible, in 

 an extraordinary degree, of various impressions 

 of light : that it will be sensible to an object illu- 

 minated as one, and as thirty thousand. It is ob- 

 vious, that either we must have groped in the dark 

 during the evening or moonlight, or have been 

 quite dazzled and overpowered by the brightness 

 of the sun, had not this fine mechanical apparatus 

 of the iris been adapted and assigned for the pro- 

 tection of the nerve. 



But the nerve is protected in another way, or 

 rather, we should say, the force of the impression 

 is regulated. We have seen that the colours of 

 objects are owing to the rays of light being reflect- 

 ed from them; that on a surface perfectly black 

 the rays sink in and are lost, and we recognise 

 the object only by its outline being contrasted with 



