XXVI. Oh a Change in the State of an Eye affected with a Mal-formation. 

 By G. B. Airy, Esq., Atstronomer Royal. 



[Read Jlay 25, 1846.] 



Twenty years ago, I had the honour of submitting to this Society a statement of the effects of 



a mal-formation in my own left eye. The nature of the effect was this: that the rays of light 



coming from a Uimiiious point and falling upon the whole surface of the pupil do not converge 



to a point at any position within the eye, but converge in such a manner as to pass through 



two lines at right angles to each other, (a geometrical phenomenon, to which the term astigmatism 



was very happily affixed by the present Master of Trinity College), and that these lines, in the 



ordinary position of the head, are both inclined to the vertical in the manner described in my 



paper {Cambridge Pliilosophical Transactions, Vol. II.) The evidence of this astigmatism, and 



the measure of it, are given by the simple observation of bringing the luminous point nearer 



and nearer to the eye ; the lines of focal convergence, according to the usual rules of focal 



position, move in the same direction in the interior of the eye ; and thus one line and the other 



line are successively brought Upon the retina ; or the image of the point becomes successively 



a line in one direction or in the other direction, these directions being at right angles to each 



other. It was found in 182.'i that the distances at which the luminous point must be placed to give 



linear images were 3'5 and 6"0 inches ; and the difference of the reciprocals of these numbers, or 



O-lig, is a proper measure of the astigmatism. The fault of the eye was corrected, as regards 



the production of distinct vision, by the use of a lens of which one surface was spherically 



concave, and the other surface cylindrically concave, and the radius of the cylindrical surface 



was such as to give a power 0"U9, or, in combination with a plane surface, to give a focal length 



1 . , . . . , n-\ 



— inch, or it was in inclies . 



0119 0119 



Some years since, I found, from some unrecorded observations, that the general short-sighted- 

 ness of the eye had sensibly altered, but that the measure of astigmatism remained nearly the same 

 as at first. 



Lately, having found that the spectacles constructed for me in 1825 do not very well suit the 

 present condition of the eye, I have made observations in precisely the same manner as in 182.'), by 

 viewing a very fine hole pricked in a card, and causing that card to slide upon a scale whose end 

 rests upon the orbital bone of the eye, and measuring the distances at which the card is placed when 

 the point aj)pcars as a line. I have been careful to hold the body and head in the same general 

 position as before: the accuracy of the measures being sensibly affected by these circumstances. 



As far as I can remember, tlie indication of the focal lines to the horizon, their length, and tiieir 

 sharpness, are not in the smallest degree changed. But the distances of the luminous point which 

 produce them are sensibly changed. They were formerly 3-5 and C'O inches : they are now -l-y and 

 8'9 inches. The eye therefore has become generally less short-sighted than it was formerly. 



But the measure of the astigmatism, which was formerly 



II .11 



= 0-119, 18 now = 0-100. 



3-5 6-0 4-7 8-9 



Vol.. VIII. Paut III. 3 A 



