EKECT IMPRESSIONS PRODUCED BY INVERTED IMAGES. 801 



dent that convergence of the eyes always occurs in looking at very near objects. It 

 becomes a question, then, whether the contraction of the pupil in accommodation for near 

 objects be associated with the action of the third nerves, or with filaments from the 

 ophthalmic ganglion, which supplies the nervous influence to the ciliary muscle. This 

 seems to have been definitively settled by Donders, who demonstrated two important 

 points: First, that increased convergence of the visual lines without change of accommo- 

 dation makes the pupil contract, as is easily proven by simple experiments with prismatic 

 glasses. Second, that when accommodation is effected without converging the visual 

 axes, "each stronger tension is combined with contraction of the pupil." 



The action of the iris, as is evident from the facts just stated, is to a certain extent 

 under the control of the will ; but it cannot be disassociated, first, from the voluntary 

 action of the muscles which converge the visual axes, and second, from the action of the 

 ciliary muscle. Donders states that, by alternating the accommodation for a remote and 

 a near object, he could voluntarily contract and dilate the pupil more than thirty times 

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

 tions 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 accommodation with muscular action, cases 

 are cited in works on ophthalmology, in which there is paralysis of the ciliary mus- 

 cle as well as cases in which the act of accommodation is painful. 



An interesting phenomenon connected with accommodation is observed in looking at 

 a near object through a very small orifice, like a pinhole. The shortest distance at 

 which we can see a small object distinctly is about five inches ; but, if we look at the 

 same object through a pinhole in a card, it can be seen distinctly at the distance of about 

 one inch, 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 is probably due to the fact that its dis- 

 tance from the eye is many times less than the distance at which distinct vision is possible 

 under ordinary conditions. 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 

 extraordinary distinctness. 



Erect Impressions produced by Images inverted upon the Retina. 



If we have become thoroughly acquainted with the mechanism of the formation of 

 images upon the retina and the physiological action of the different parts of the optical 

 apparatus, it will be sufficient to note the action of both eyes, as contrasted with the 

 action of one, in normal vision, without discussing fully the multitude of curious observa- 

 tions made with the stereoscope ; and we can readily comprehend the action of muscles by 

 which the axis of vision is directed toward different objects, without entering into a discus- 

 sion of abstruse mathematical calculations with regard to the exact centre of rotation, the 

 law of torsions, and other points connected with physiological optics. These are ques- 

 tions, however, of great interest to ophthalmologists and are fully discussed in elaborate 

 special treatises. 



We shall allude briefly, in this connection, to a question which has long engaged the 

 attention of physiologists, and one which, we cannot but think, has been made the sub- 

 ject of much unprofitable speculation. It is a matter of positive demonstration that the 

 images of objects seen are inverted as they appear upon the retina. Why is it, however, 

 that objects are appreciated as erect, when their images are thus inverted ? With a 

 knowledge of the fact that the appreciation of impressions made upon the nerves of 

 special sense is capable of education and is corrected by experience, it seems hardly 

 necessary to enter into an elaborate discussion of this point. We appreciate with accu- 

 racy the density of objects, the direction of sounds, differences in musical tones, the 

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