354 LECTURE XXXVIII. 



The iris serves, by its variable magnitude, to exclude more or less of the 

 light falling on the cornea, when its intensity would otherwise be too great ; 

 hence the pupil is usually smallest by day, and its increased magnitude at 

 night sometimes gives the eye a greater apparent lustre. The iris also inter- 

 cepts such rays as would fall on parts incapable of refracting them regu- 

 larly ; and by its contraction when a nearer object is viewed, it lessens the 

 confusion which would arise, in such eyes as cannot accommodate them- 

 selves sufficiently, from the magnitude of the imperfect focal points on the 

 retina. Such a contraction almost always accompanies the diminution of 

 the focal length, even in a perfect eye, and it may easily be rendered visible 

 by walking gradually up to a looking glass, and observing the magnitude 

 of the pupil as we approach nearer and nearer to our image. It would be 

 difficult to assign a reason for this change of the state of theTpupil within 

 the limits of perfect vision, unless we allowed the irregularity of the form 

 assumed by the marginal parts of the crystalline lens. The iris is also 

 peculiarly useful in excluding such parts of lateral pencils of light as 

 fall very obliquely on the cornea, and are too much refracted, while a 

 smaller pencil only, which enters the eye more directly, is admitted into 

 the pupil. 



The refractive powers and properties of the eye may be very conveniently 

 ascertained by means of an instrument to which I have given the name 

 optometer, a term first employed in a sense nearly similar by Dr. Porter- 

 field.* If two or more separate parcels of the rays of the same pencil be 

 admitted at distant parts of the pupil, they will only be reunited on the 

 retina when the focus is perfect, so that if we look through two small per- 

 forations, or slits, at a minute object, to the distance of which the eye is 

 not accommodated, it will appear as if double ; and when the object is a 

 line directed nearly towards the eye, each point of it will appear double, 

 except that which is at the distance of perfect vision, and an image of two 

 lines will be seen, crossing each other in this point ; so that the measure- 

 ment of the focal length of the eye is immediately performed by inspection 

 of the optometer only. The scale may be extended by the addition of a 

 lens, which enables us to produce the effect of a longer line, while the 

 instrument still remains portable. 



When the eye is possessed of too great a refractive power for the distinct 

 perception of distant objects, the pupil is generally large, so that the confu- 

 sion of the image is somewhat lessened by partially closing the eyelids ; 

 and from this habit an eye so formed is called myopic. In such cases, by 

 the help of a concave lens, the divergence of the rays of light may be 

 increased, and a virtual image may be formed, at a distance so much 

 smaller than that of the object as to afford perfect vision. For a long 



Bat. 1746. Le Roy, Mem. sur la M6e. par lequel 1'CEil s'Accommode, Hist, et 

 Mem. 1755, p. 594. Gibers, De Oculi Mutationibus Intends, 4to, Gott. 1780. 

 Young, Ph. Tr. 1793, p. 169 ; 1801, p. 23. De Corp. Hum. Viribus Conserva- 

 tricibus, Gott. 1780. Hunter, Ph. Tr. 1794, p. 21. Home, ibid. 1800, p. 146. 

 Brewster in Ed. Jour, of Science, i. 77. Treviranus, zur Anat. der Sinneswerk- 

 zeuge, 1828. Kolrausch on Treviranus' Hypoth. 1837. Luchtman, De Mutatione 

 Oculi, Tr. ad Rhenum, 1832. Simonoff, Jour, de Physiol. iv. 260. 

 * Edinb. Med. Essays, iv. 185. 



