I4-G8 



VISION. 



vision, a point appearing a line, a circle an 

 oval, and a square a parallelogram. In an in- 

 teresting case related by Dr. Robert Hamil- 

 ton*, the patient, when looking at a clock, was 

 unable to distinguish the hands if they pointed 

 perpendicularly, as at six o'clock, but if hori- 

 zontally he had no difficulty : so when looking 

 at a wheel at a little distance, the horizontal 

 spokes only could be seen. The patient was 

 a coach painter by trade, and this peculiarity 

 of vision greatly interfered with his business, 

 for he could not draw vertical lines with any 

 degree of correctness, and unwittingly made 

 them slanting, a serious fault in heraldic de- 

 vices ; horizontal lines he drew with perfect 

 precision. His method of correcting the per- 

 ceptions of perpendicular lines was to bend 

 his head at right angles with his body, where- 

 upon upright bodies became distinct and accu- 

 rately represented. This man also practised 

 a manoeuvre which forcibly reminds us of an 

 act common to persons having conical corneae, 

 that of placing the fore-finger at the outer 

 angle of the eyelids and drawing them out- 

 wards whereby vision is improved. 



To remedy the defect under which he 

 laboured, Professor Airey made a pin-hole in 

 a blackened card, which he caused to slide on 

 a graduated scale ; then strongly illuminating 

 a sheet of paper and holding the card between 

 it and the eye, he had a lucid point on which 

 he could make observations with ease and 

 exactness. Then resting the end of the scale 

 on the cheek bone he found that the point at 

 the distance of 6 inches appeared a very well- 

 defined line inclined to the vertical about 35 

 and subtending an angle of 2. Again, at the 

 distance of 3^ inches, it appeared a well-defined 

 line at right angles with the former, and of the 

 same apparent length. It was therefore neces- 

 sary to make a lens which, when the parallel 

 rays were incident, should cause them to 

 diverge in one plane from the distance of 3J 

 and in the other plane from the distance of 

 6 inches. The professor obtained a lens of 

 which the radius of the spherical surface was 

 3^- inches, of the cylindrical 4 inches, and with 

 this he was able to read the smallest print. 



In Dr. Hamilton's patient the relation of the 

 horizontal to the vertical focus appeared to be 

 as 5i inches to 6| inches, and on trying him 

 with plano-concave cylindrical lenses, it was 

 found that a lens of 24 inches focal length, 

 the cylindrical surface being made to act ho- 

 rizontally, operated very beneficially. Besides 

 this irregular refraction the man was myopic, 

 but the lenses in question fitted as spectacles, 

 enabled him to see well. 



The defect of the cylindrical eye may be de- 

 tected by making a small pinhole 'in a card 

 which is to be moved from close to the eye to 

 arm's length, the eye meanwhile being directed 

 to the sky, or any bright object of sufficient 

 size. With ordinary eyes the indistinct image of 

 the hole remains circular at all distances, but 

 to an eye having this peculiar defect it be- 

 comes elongated, and when the card is at a 



* Monthly Journal of Medical Science, June 1847. 



certain distance passes into a straight line. 

 On further removing the card the image be- 

 comes elongated in the perpendicular direc- 

 tion, and finally if the eye be not too 

 long-sighted, passes into a straight line per- 

 pendicular to the former. 



Professor Stokes has invented a highly 

 ingenious instrument for determining the 

 nature of the required lens, and the following 

 is the proposition on which it is based. 



Conceive a lens ground with two cylindrical 

 surfaces of equal radius, one concave and the 

 other convex, with their axes crossed at right 

 angles ; call such a lens an astigmatic lens : 

 let the reciprocal of its focal length in one of 

 the principal planes be called its power, and a 

 line parallel to the axis of the convex surface 

 its astigmatic axis. Then if two thin astig- 

 matic lenses be combined with their axes 

 inclined at any angle, they will be equivalent 

 to a third astigmatic lens determined by the 

 following construction. 



From any point draw two straight lines 

 representing in magnitude the powers of the 

 respective lenses, and inclined to a fixed line 

 drawn arbitrarily in a direction perpendicular 

 to the axis of vision at angles equal to twice 

 the inclinations of their astigmatic axes, and 

 complete the parallelogram. Then the two 

 lenses will be equivalent to a single astigmatic 

 lens represented by the diagonal, of the pa- 

 rallelogram in the same way in which the 

 single lenses are represented by the sides. 

 A piano-cylindrical or spherico-cylindrical 

 lens is equivalent to a common lens, the 

 power of which is equal to the semi-sum of 

 the reciprocals of the focal lengths in the two 

 principal planes, combined with an astigmatic 

 lens, the power of which is equal to their 

 semi -difference. If two piano -cylindrical 

 lenses of equal radius, one concave and the 

 other convex, be fixed one in the lid and the 

 other in the body of a small round wooden 

 box, with a hole in the top and bottom, so as 

 to be as nearly as possible in contact, the 

 lenses will neutralise each other when the 

 axes of the surfaces are parallel ; and by 

 merely turning the lid round, an astigmatic 

 lens may be formed of a power varying con- 

 tinuously from zero to twice the astigmatic 

 power of either lens. When a person who 

 has the defect in question has turned the lid 

 till the power suits his eye, an extremely 

 simple numerical calculation, the data of 

 which are furnished by the chord of double 

 the angle through which the lid has been 

 turned, enables him to calculate the curva- 

 ture of the cylindrical surface of a lens for 

 a pair of spectacles which will correct the 

 defect in his eye.* 



A curious case is related in the AnnaN 

 d'Oculistiquef, of an anomaly of vision, 

 which was probably the consequence of a 

 defect in the form of the cornea, such as 

 that under consideration. M. Schnyder, the 

 Pastor of Menzberg in the Canton of Lucei 



* Report of the British Association, vol. xviii,- 

 f Tom. xxi. p. 222. 



