Mr. GOODE, on A PECULIAR DEFECT OF VISION. 495 



wards, without much affecting the position of the resulting images. The other inconvenience is, that 

 the eye is naturally endowed with a power of adapting itself to different distances, and that this 

 power is very little under command in an eye which is not habitually used, a circumstance, perhaps, 

 frequent with those who have one eye defective : such an eye, when tested by any instrument, will 

 at one instant appear to have one focal length, and at another instant another. When a cross drawn 

 on paper is held at a distance between the two foci, I find that I can at will discern either the 

 horizontal line, or the perpendicular, without altering the position of the paper. 



There are, therefore, no means of attaining the requisite measurements beyond an approximation, 

 and the rest must be ascertained by direct experiment with a series of cylindrical lenses. 



Having calculated an approximation of the glass I required, I applied to M. Chamblant, 

 a working optician at Paris, who occupies himself solely with the construction of lenses and 

 spectacle glasses with cylindrical surfaces, and after several trials I succeeded in obtaining a glass, 

 which gives me distinct vision of objects both far and near alike, thus shewing that the error of 

 malformation is independent of the state of adaptation of the eye. The glass I use is piano- 

 cylindrical, the cylindrical surface concave, with a radius of curvature of nine French inches. The 

 axis of the cylinder when presented to the eye, coincides of course with the direction of the line at 

 the nearer focus. 



A plano-convex glass also, with the axis perpendicular to the direction of the line at the first 

 focus, and the curvature of which is the same, gives distinct vision, provided that the object 

 is placed sufficiently near to the eye; or even a glass tnuch stronger, when the object is very 

 close to the eve. 



Considering that the inclination of the lines at the foci might have a physiological importance, 

 I devised the following method of determining it accurately. If a number of pinholes be pricked 

 in a card, in a straight line, and the card be fixed in such a manner that it may be made to revolve, 

 and have an illuminated surface behind it, when a defective eye is placed at the proper distance, it 

 readily recognizes the position of the card in which all the lines representing the images of the 

 pinholes lie in one straight line, being the line in which the holes are pricked : care must be 

 taken that the body is held perpendicularly. It is easy now to determine the inclination of this 

 line to a hair stretched vertically by a weight. 



Within the last few months I have met with three or four cases of defective vision similar 

 ito my own ; only two of which are of sufficient magnitude to be worthy of mention. 



One is that of Mr. Parry, wlio has served many years as a medical officer in the army. This 

 I gentleman's left eye is perfect, except in being somewhat presbyopic, but from the time of his 

 1 earliest recollection he has never had distinct vision with his right eye ; he has never been able 

 I to read with it, though he has an indistinct vision of objects at all distances. 



Ilis eye, tested by a pinhole in a card, perceived the hole as a horizontal line at the distance 

 of .S? centimetres (about 14l inches); tiie line is inclined at an angle of 87 degrees to the 

 mesial plane of the body, and meets this plane produced inwards and upwards. At some distance 

 beyond this the hole appeared enlarged, and of a rhomboidal figure, but never as a line. 



Wlien he viewed two lines drawn in the form of a cross, he saw well enough the horizontal 

 jline at 1 4-| inches, and for some distance beyond, but at no distance could he discern the vertical 

 lline. The error therefore seemed to consist in an exceedingly feeble refractive power in liorizontal 

 planes: I therefore tested his eye with piano-cylindrical convex glasses, in order to obtain data 

 per calculating the forms of glasses to be used for viewing objects at different distances ; and 

 jwe found that with a glass of 2^ French inches radius, the two lines of the cross, at 12 or 14 inches 

 Idistance, appeared of nearly equal brightness. This glass was rather too strong, while 3 inches 

 Igave a glass rather too weak. To view distant objects, therefore, I caused to be nuule a glass 

 Icylindrical concave on one side, with a radius of 7^ French inches, cylindrical convex on the 

 [other, with a radius of 4^; the axes of the cylinders of course crossing at right angles, and 



Vol.. VIII. Pabt IV. :s S 



