494 Mr. GOODE, on A PECULIAR DEFECT OF VISION. 



7. On viewing two dark or two bright lines drawn in the form of a cross, and held in the 

 position of the lines above indicated, the vertical line appears broad and very faint at the distance 

 of 6.13 inches, the horizontal line appearing clear and well defined ; while the reverse is the case 

 beyond 25 inches. 



There is no apparent defect in the left eye. If several holes be pricked near one another, and 

 viewed by this eye at the distance of 5 inches, which is somewhat within the range of distinct vision, 

 a dark central spot is observable in the centre of each ; also a narrow luminous slit appears traversed 

 in the direction of its length by two central parallel dark lines. 



It is probable that the defect of the eye is inherited, as my mother has a defect of a similar 

 nature in both eyes. A circular pinhole viewed with either eye at the distance of 7-5 inches appears 

 as an ellipse with the major axis parallel to the mesial plane of the body, while at the distance 

 of 5 inches the image is an ellipse with the major axis perpendicular to the former. 



Since the period when the above measurements were taken, I have made frequent use of the 

 eye; owing, most probably, to this circumstance, a very considerable amelioration has taken place in 

 it ; the first focus, which in the month of June last was at 6.13 inches, in the month of December 

 was at about 10: the second focus was readily ascertained in the month of June to be at between 

 2-1 and 25 inches ; but in December it was impossible to determine the exact position of it by the 

 simple observation of a pinhole ; because, instead of appearing as a sharp distinct line, as before, the 

 image was always confused by the presence of the luminous square above described. The image 

 was, in fact, a rhomb, with the longer diagonal, distinguished by its brightness, in the direction 

 of the further line, while the line seen at the nearer focus never disappeared, but became shortened, 

 remaining always the brightest part of the image, and forming the shorter axis of the rhomb. How- 

 ever, by means of the instrument described below, the second focus was ascertained to be at a 

 distance of between 27 and 28 inches. Since December, up to the month of March of this year, 

 no change whatever has taken place in the eye, notwithstanding the constant use of it. 



The length of the line, as observed at either focus, is, of course, dependent on the aperture of 

 the pupil, and the distance from the retina, before or behind it, of the line of convergence of the rays 

 refracted from the other focus. 



The differences in the eye observed in June and December, are exactly such as occur, when 

 similar observations are made on a sound eye, to which is applied, in one case, a cylindrical convex 

 lens of short focal length, and in the other a lens of weaker power. 



The instrument above alluded to as serving to determine the distance of the foci is simply that 

 of Scheiner. Let a tube which slides within another in the manner of a telescope be closed at 

 its extremity by a card pierced by a single pinhole, while the other extremity of the apparatus 

 is closed by a card pierced by two holes, the distance of which from one another is less than the 

 diameter of the pupil of the eye of the observer. When the extremity pierced by the single hole is 

 presented towards a luminous surface, and the other is applied close to a sound eye, if the distance 

 of the single hole is equal to the most convenient distance of distinct vision, free from any exertion, 

 the hole will appear single ; but if the distance be greater or less than this, the hole will appear 

 as two ; as is well known. This instrument may be applied to the determination of the two foci of 

 a defective eye, by observing that, in order to ascertain the distance of either focus, the line passing 

 through the two pinholes must be perpendicular to the direction of the line, which forms the image 

 of tlie point at that focus. 



There are, however, two inconveniences attending the use of this instrument ; namely, firstly, 

 that if the eye, on which the observation is made, be at all long-sighted, so that the pinhole requires 

 to be placed at a considerable distance, the two pencils of light falling on the two pinholes are nearly 

 parallel, and thei'efore the pinhole may be moved through a considerable space backwards and for- 



