SCIENCE AND POETRY. 



consistent with physical truth, without being less beautiful, than 

 that which we have quoted above. 



How happily, for example, did he avail himself of that beautiful 

 property of the iris by which it accommodates the eye to greater and 

 less degrees of light, enlarging the pupil when the light is faint, 

 and contracting it when it is intense. 



The iris, as is well known, is the coloured ring which sur- 

 rounds the dark spot in the middle of the eye ; this dark spot 

 being not a black substance, but a circular orifice through which 

 the light is admitted to the membrane lining the posterior part 

 of the internal chamber of the eye. This circular orifice is called 

 the pupil, the retina being the nervous membrane which pro- 

 duces the visual perceptions. These, with other particulars of the 

 structure of the eye, will be found fully explained in our Tract on 

 that subject. 



The iris which surrounds the pupil has a certain power of con- 

 traction and expansion, which is produced by the action upon it of 

 proper muscles provided for that purpose. 



The quantity of light admitted through the pupil to the retina 

 is increased or diminished in the proportion of the area of the 

 pupil, which increases and diminishes in proportion to the square 

 of its diameter ; a very small variation of which therefore pro- 

 duces a very considerable proportionate variation of the quantity 

 of light admitted. 



If a person, after remaining for some time in a room dimly 

 lighted, pass suddenly into one which is strongly illuminated, he 

 will become instantly sensible of pain in the retina, and will 

 involuntarily close his eyes. After a short time, however, he will 

 be enabled to open them and look around with impunity. 



The cause of this is easily explained. In the dimly lighted 

 room the pupil was widely expanded to collect the largest quantity 

 possible of the faint light, so that a sufficient quantity might be 

 received by the retina to produce a sensible perception of the 

 surrounding objects. On passing into the strongly illuminated 

 room the expanded pupil admits so much of the intense light as 

 to act painfully on the retina, before there is time for the iris to 

 adjust itself so as to contract the aperture of the pupil. After 

 a short interval, however, this adjustment is made, and the area 

 of the pupil being diminished in the same proportion as the 

 intensity of the light to which it is exposed, has been augmented 

 in passing from the one room to the other, the action upon the 

 retina is proportionally mitigated, so that the eye can regard 

 without pain the surrounding objects. 



The reverse of all this takes place when the eye suddenly 

 passes from strong to feeble illumination. The pupil contracted 

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