VISUAL ACCOMMODATION. Yll 



accommodation is effected without converging the visual axes, " each stronger 

 tension is combined with contraction of the pupil." Contraction of the 

 pupil, therefore, occurs both in convergence of the visual axes without ac- 

 commodation and in accommodation for near objects without convergence of 

 the eyes. 



The action of the iris, as is evident from the facts just stated, is to a cer- 

 tain extent under the control of the will ; but it can not be disassociated, 

 first, from the voluntary action of the muscles which converge the visual 

 axes, and second, from the action of the ciliary muscle. Bonders, by alter- 

 nating the accommodation for a remote and a near object, was able to volun- 

 tarily contract and dilate the pupil more than thirty times in a minute. 

 Brown-Sequard, in discussing the voluntary movements of the iris, has men- 

 tioned a case in which " the pupil could be contracted or dilated without 

 changing the position of the eye or making an effort of adaptation for a long 

 or a short distance." As a farther evidence of the connection of accommoda- 

 tion with muscular action, cases are cited in works on ophthalmology, in 

 which there is paralysis of the ciliary muscle, as well as cases in which the 

 act of accommodation is painful. 



A curious phenomenon connected with accommodation may be observed 

 in looking at a near object through a very small orifice, like a pinhole. The 

 shortest distance at which one can see a small object distinctly is about five 

 inches (12'7 centimetres) ; but in looking at the same object through a pin- 

 hole in a card, it can be seen distinctly at the distance of about one inch 

 (25'4 mm.), and it then appears considerably magnified. In this experiment, 

 the card serves as a diaphragm with a very small opening, so that the centre 

 of the lens only is used ; and the apparent increase in the size of the object 

 probably is due to the fact that its distance from the eye is many times less 

 than the distance at which distinct vision is possible under ordinary condi- 

 tions. It is well known that myopic persons, by being able to bring the eye 

 nearer to objects than is possible in ordinary vision, can see minute details 

 with peculiar distinctness. 



Erect Impressions produced by Images inverted upon the Retina. The 

 images which make visual impressions are necessarily inverted upon the 

 retina ; but the cerebral visual centre takes no cognizance of this, and objects 

 are seen in their actual position. It seems almost absurd to enter into a seri- 

 ous discussion of this fact. In the words of Helmholtz, " our natural con- 

 sciousness is completely ignorant even of the existence of the retina and of 

 the formation of images : how should it know any thing of the position of 

 images formed upon it ? " 



Field of Indirect Vision. If the eye be kept fixed upon a certain point, 

 and an object be moved from this point as a centre in lines radiating in dif- 

 ferent directions until it passes from the field of view, the limits of indirect 

 vision are indicated. Eight or ten such points of limit, connected by a 

 curved line, give a map of the visual field. This may be done roughly upon a 

 fiat surface, such as a blackboard, placed at a distance of twelve to eighteen 

 inches (3 to 4'5 centimetres) from the eye, or a chart may be made with an 



