OPtlCS. 



45 



Hitherto we have considered the eye 

 as a single organ, and as viewing ob- 

 jects at that precise distance when rays 

 diverging from them are converged to 

 points on the retina ; but as almost all 

 animals have more than one eye, and 

 have the power of seeing objects dis- 

 tinctly at different distances, we must 

 proceed to the consideration of these 

 two points. 



9. Cause of single vision with two 

 eyes. The subject of single vision with 

 two eyes has excited much needless 

 discussion, as it is the necessary con- 

 sequence of the law of visible direction. 

 By the external muscles of the eyeball we 

 can direct the axis of each eye, so that 

 these axes when prolonged may meet 

 in any point of absolute space beyond 

 the distance of four or five inches. Let 

 us suppose that we are placed at one end 

 of a room, and that we direct the axes 

 of both eyes to a circular aperture in a 

 window shutter at the other end ; then, 

 though an image of this aperture is 

 formed in each eye, yet because the 

 lines of visible direction from similar 

 points of the one image meet the lines 

 of visible direction from similar points 

 of the other image, each pair of simi- 

 lar points must be seen as one point, and 

 the aperture seen by one eye will exactly 

 coincide with the aperture seen by the 

 other eye. The same singleness of vision 

 would take place if we possessed an 

 hundred eyes, all capable of having 

 their axes directed to the same point. 



If when an object is seen single with 

 both eyes, we press one eye aside, the 

 image formed by that eye will separate 

 from the other image, and the object 

 will appear double. Or, if the axes of 

 both eyes are directed to a point either 

 nearer or more remote than the aperture 

 in the window shutter, then in both of 

 these cases the aperture will appear dou- 

 ble ; because the similar lines of visible 

 direction no longer meet at the aperture. 



A small object may, and sometimes 

 does, appear double with one eye, when 

 the crystalline lens has ceased to be ho- 

 mogeneous, either from age or disease. 



10. Cause of squinting. A person is 

 said to squint when both eyes do not 

 seem to be directed to the object at 

 which he is looking. When either of the 

 eyes has a less perfect vision, or a dif- 

 ferent focal length, or when there is any 

 weakness in its external muscles we are 

 apt to make use only of the good eye ; 

 and when we acquire the habit of cluing 

 this the imperfect eye is left at rest, and 



will sometimes cease 'even to follow the 

 movements of the other. In this case 

 squinting is produced. If the good 

 eye is shut, and the bad one forced 

 to exert itself, the iris will be placed 

 symmetrically between the eyelids, and 

 the squint formerly seen in the eye will 

 disappear. Should the eye in this 

 case still squint, the cause of it must 

 be sought either in the central hole of 

 the retina not being at the extremity of 

 the axis, or in some mal-conformation 

 by which the retina is not perpendicular 

 to the axis of the eye at the point where 

 they meet. Such a case we have never 

 met with. This disease of the eye 

 might, we are persuaded, be frequently 

 cured, even in adults, by those who are 

 thoroughly acquainted with the struc- 

 ture and functions of this organ. 



11. Accommodation of the eye to dif- 

 ferent distances. The narrowness of 

 our limits will not permit us to detail 

 the various theories which have been 

 devised to explain this property of 

 the eye, and the experiments which 

 have been made to support them. The 

 eye is, we conceive, adjusted^to very 

 remote objects when it is in a state of 

 perfect repose. When near objects are 

 to be seen, we are enabled by a volun- 

 tary action to draw forward the crystal- 

 line lens. This action is performed by 

 the contraction of the pupil, or by the 

 expansion of the iris towards the centre 

 of the pupil, and as the base of the iris 

 is connected with the ciliary processes 

 which suspend the lens, the lens will be 

 thus removed from the retina. But 

 while the eye possesses the power of 

 voluntary adjustment, the same effect 

 may be produced involuntarily by the 

 stimulus of light upon the eye. By a 

 combination of the voluntary and in- 

 voluntary actions, the eye is accommo- 

 dated to all distances within its range ; 

 and for short distances, when the volun- 

 tary power of adjustment fails, the ad- 

 justment may still be effected by the 

 involuntary stimulus of light. The 

 facts and reasonings by which this 

 view of the subject is supported, will be 

 found in the Edinburgh Journal of 

 Science, No. i. p. 77 83. 



The accommodation of the eye to any 

 distance is effected at the same time 

 with the convergency of the axes of 

 both eyes to the object to be viewed. 

 These two movements being neces- 

 sarily called into action at the same 

 instant, cannot easily be performed se- 

 parately, which has led to the belief 



