278 SENSE OF SIGHT. 



of the adductor muscle being stronger, shorter, and its course more in 

 a straight line than that of any of the other muscles ; and Sir Charles 

 Bell 1 ingeniously applies his classification of the muscles of the eye to 

 an explanation of the fact. He asserts, that the recti muscles are in 

 activity whilst attention is paid to the impression on the retina, but 

 that, when the attention is withdrawn, the recti are relieved, and the 

 eyeball is given up to the influence of the oblique muscles, whose state 

 of equilibrium exists when the eyeball is turned, and the pupil pre- 

 sented, upwards and inwards. 



Lastly, in persons who are in the habit of making repeated celestial 

 observations, or in those who make much use of the microscope, the 

 attention is so entirely directed to one eye, that the other is neglected, 

 and, in time, wanders about, so as to produce squinting at the pleasure 

 of the individual. In these cases, the eyes become of unequal power ; 

 so that one only can be employed where distinct vision is required. 



Thus far our remarks have been directed to double vision, where 

 both eyes are employed. We have now to mention a singular fact con- 

 nected with double and multiple vision with one eye only. The author 

 has distinct double vision with each eye ; a lighted lamp, for example, 

 presenting to one, with the other closed, two defined images, the 

 one in advance of the other. If a hair, a needle, or any small object 

 be held before one eye the other being closed and within the point 

 of distinct vision, so that the bright light of a lamp or from a window 

 shall fall upon the object in its passage to the eye, or be reflected from 

 it we appear to see not one object but many. This fact, when it was 

 first observed by the author, appeared to him to have escaped the ob- 

 servation of opticians and physiologists, inasmuch as it had not been 

 noticed in any of the works recently published on optics or physiology. 

 On reference, however, to the excellent " system," of Smith, 2 on the for- 

 mer subject, he found in the "Essay upon Distinct and Indistinct Vision" 

 by Dr. Jurin, appended to it, the whole phenomenon explained, and elu- 

 cidated at considerable length. The elaborate character of the expla- 

 nation is probably the cause, why the fact has not been noticed by sub- 

 sequent writers. The best way of trying the experiment is that sug- 

 gested by Dr. Jurin. Take a parallel ruler, and opening it slightly, 

 hold it directly before the eye, so as to look at a window or lamp through 

 the aperture. If the ruler be held at the visual point, the aperture will 

 appear to form one luminous line ; but if it be brought nearer to the 

 eye, it will appear double ; or as two luminous lines, with a dark line 

 between them ; and according as the aperture is varied or the dis- 

 tance from the eye two, three, four, five or more luminous and dark 

 parallel lines will be perceptible. 



At first sight, it might seem, that this phenomenon should be referred 

 to the diffraction or inflection, which light experiences in passing by the 

 edges of a small body, as the hair or needle. Newton had long ago 

 shown, that, when a beam of light shines upon a hair, the hair will cast 

 several distinct shadows upon a screen, and, of course, present several 

 images to the eye. Dr. Bittenhouse 3 explains, on the same principle, a 



i Anat. and Physiol., edit, cit., ii. 235. 2 Optics, edit. cit. 



3 Amer. Philos. Transact., vol. ii. 



