XXXVI. On a Peculiar Defect of Vision. By Henry Goode, M.B., 



of Pembroke College. 



[Read November 9, 1846, and May 17, 184?.] 



The following details of a case of defective vision may not be uninteresting. 



About ten years ago I first perceived a defect of vision in the right eye, the extent of which, 

 before that period, I believe to have been inconsiderable : the defect being that small objects, 

 when viewed at the distance of greatest distinctness, appear as two. My attention havinn- been 

 called to Professor Airy's Paper on his own eye, I find that my eye, tested in the manner he 

 proposes, exhibits a similar defect. This method is to view with the defective eye a pinhole in 

 a card, which slides along a graduated scale, one extremity of the scale being applied to the cheek- 

 bone, and the other directed towards an illuminated sheet of paper. 



The following are the appearances observed : 



1. When the card is quite close to the eye, the image of the pinhole is perfectly circular. 



2. As the card is removed to a greater distance, the image becomes gradually elongated 

 in the form of an ellipse, with a sharp dark line in the long diameter, most distinct at the distance 

 of -1.5 inches, and best visible in a minute hole. 



3. At 6.13 inches the image has become extended into a bright well-defined line, of the breadth 

 of the pinhole as estimated by the sound eye, and crossed in the centre by a dark line perpendicular 

 to the former dark line which has disappeared : if several pinholes be pricked near one another, the 

 dark band holds the same relative position in all of them. 



4. As the card is removed to a further distance, the bright line becomes gradually shortened, 

 and at the distance of more than a foot appears as two bright spots only, situated one on each side 

 of the dark band ; but, at the same time, in the direction of, and as it were overlying the dark band, 

 a bright line gradually appears, short at first, and becoming elongated with the removal of the card, 

 so that at about 10 inches or more the appearance is that of a cross, most strongly illuminated in 

 the position of the two bright spots before described. At 12.2 inches this cross appears as a regular 

 quadrangular figure with concave sides, the two spots being most strongly illuminated. If a dark 

 spot on a sheet of white paper be viewed in the same manner, the appearance is necessarily the 

 same; but owing to the greater distinctness of the two spots, the remainder of the figure is easily 

 overlooked, and the appearance is that of a double spot ; consequently, if a page of small type be 

 viewed at this' distance, the print appears double. 



.5. When the card is at 25 inches, and all greater distances, the image is a bright line perpen- 

 dicular to that seen at 6.1, 'J inches, the two spots representing that line having almost coalesced into 

 one, causing the bright line to be brightest in the centre. 



Distant luminous objects with clear defined outlines, such as the Moon, apj)enr as a succession of 

 well-defined images overlying one another with their centres in this line. 



6. The more distant line is inclined to the mesial plane of tlic body at an angle of 21", and the 

 upper part falls inwards towards this ])lane. 



It appears in the above, that a short distance within tiie nearer focus a dark line occupies the 

 position of the bright line seen at that focus, while beyond the focus at all distances tiie line continues 

 illuminated. The same holds with regard to the second focus. 



