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DIOPTEIC MECHANISM OF THE EYE 313 



power of the eye brought about by the activity of the muscles of 

 the ciliary body, the term moderative accommodation may be 

 applied to the changes of the aperture^ of the pupil consequent on 

 the alterations of tone of the iridic muscles. 



The movements of the pupil are involuntary and unconscious 

 reflex acts. Under normal conditions the pupils are of equal size, 

 and react reflexly to different stimuli synchronously and identi- 

 cally. 



The width of the pupil when the degree of illumination is 

 moderate differs greatly in different people, particularly in relation 

 to age. It is very narrow in the new-born ; increases up to 6 

 years of age ; at 20 attains a diameter of about 4 mm. ; then 

 grows increasingly less with age up to about 50, when it is usually 

 3 mm. ; and in old age is once more of minimal diameter (Silber- 

 kuhl and Pfister). The pupil is a little larger in women than 

 in men ; in hypermetropic persons it is smaller, in myopes larger, 

 up to the twentieth year than in emmetropes. These differences 

 are independent of the colour of the pigment of the iris (Tange, 

 1902). 



Reflex contraction of the pupil takes place 



(a) When the retina is excited by the incidence of light. It 

 contracts in proportion to the intensity of the light and the extent 

 of retinal surface illuminated. This reflex action of light is, up to 

 a certain point, independent of the activity of the apparatus for 

 accommodation. Even when distant objects are focussed, the 

 pupils may be contracted (myosis) with the lids half-closed when 

 the light is too intense. In the same way they may be dilated 

 (mydriasis) even in focussing near objects, if the light is weak. 



Contraction does not occur instantaneously but begins, accord- 

 ing to Listing, 0'4-0'S sec. after the incidence of light, and reaches 

 its maximum about O'l sec. later. The flash-light photographs 

 by Claude du Bois-Eeymond (1888) showed that the pupil is 

 enormously dilated in total darkness. It is therefore owing to 

 the action of light that it is considerably contracted under ordinary 

 conditions. 



The reaction of the pupil to light is in direct ratio with the 

 intensity of the source of light. According to a law formulated 

 by Vervoot (1900) an object = 1, illuminated with an intensity 

 = 4, produces the same degree of reaction in the pupil as would 

 be produced by an object four times greater, but four times more 

 weakly illuminated. The reflex must therefore depend on the 

 amount of light and not on the extent of retinal surface 

 illuminated. 



According to Ovio (1905) the size of the pupil varies inversely 

 with the square root of the intensity of the light. It has been 

 disputed by different authors whether on increasing or diminishing 

 the intensity of light the alterations in the width of the pupil 



