DIOPTRIC MECHANISMS OF THE EYEBALL 551 



with an illumination of 50 to 100 candles the size of the pupil was 3-7 mm. 

 and with 500 to 1000 candles, 3-3 mm. This effect was obtained by a rapid 

 change of the illumination of the eye. When the change in illumination is 

 sufficiently slow no alteration of the pupil takes place, and when the illumina- 

 tion, which has at first caused a maximal constriction of the pupil, is con- 

 tinued the pupil gradually relaxes with the adaptation of the retina to light. 

 This relaxation occurs within three or four minutes after exposure to light 

 has taken place. The same influence of adaptation will be observed if two 

 individuals are brought into a moderately lighted room, one from bright 

 daylight and the other from a dark room. The pupils of the first will dilate 

 widely, while those of the second will constrict to their maximum extent. 

 In each case the change will pass off gradually, so that at the end of five or 

 ten minutes there will be no difference observable between the eyes of the 

 two persons. 



(2) When vision is directed to a near object the contraction of the 

 ciliary muscle which results in accommodation is accompanied by con- 

 vergence of the visual axes, brought about by contraction of the two internal 

 rectus muscles. With these two movements is always associated a third, viz. 

 contraction of the sphincter pupillae, the increased sharpness of the image 

 obtained by this means being an indispensable condition for the fineness of 

 vision which we desire when we examine any object closely. The fact that 

 the amount of light which will fall into the eye from any given object in- 

 creases inversely as the square of the distance of the object from the eye 

 ensures that sufficient light will pass through the constricted pupil for the 

 appreciation of the finer details of the object. 



(3) In sleep the pupils are always contracted. This fact seems at first 

 in opposition to the other conditions regulating the size of the pupil, since 

 during sleep no light is falling into the eye. If the eyelid of a sleeping person 

 be raised the pupil will be found to be constricted ; as the person wakes up, 

 in consequence of the interference, the pupil dilates and may then constrict 

 again if the light is held so as to fall into the eye. This behaviour of the 

 pupil may enable us to distinguish feigned from real sleep. The constriction 

 of the pupil is really, like that which accompanies accommodation, an 

 associated condition, and depends on the fact that during sleep the axes of 

 the eyeballs are directed upwards and inwards. 



(4) Contraction of the pupil is a marked effect of the action of certain 

 drugs, especially opium and its alkaloid, morphia, as well as of the alkaloids 

 eserine, or physostigmine, and pilocarpine. Contraction of the pupil also 

 occurs in general excitatory conditions of the central nervous system and is 

 therefore found during the stage of induction of chloroform and ether 

 anaesthesia. 



Dilatation of the pupil occurs : 



(1) On the removal of light stimulus from the eye. If the removal 

 is complete the pupil remains dilated, but if there is any light at all the 

 pupil gradually constricts again as the eye becomes dark-adapted. 



(2) Dilatation of the pupil can be reflexly excited by the stimulation of 



