ON TiiE LIMITS OF SINGLE VISION. JOQ 



parallel with its vertical axis, while its centre corresponds 

 with the summit of the optic axis. 



This general deiermination of the form of the field of Attempts to 

 ,'. . ., • cc • 4. 1 J .. ascertain its^ 



Single vision appeannj^ to rne insuHicient, 1 made some at- extent. 



tempts, to point out its limits with more precision. The 

 method 1 empIo\ed consisted in comparing the angle form- 

 ed by the double image with the angle we are objiged to 

 form by the inclination o*" the optic axes to produce it. I 

 have fjund, thot ibe apparent separation of tlie double 

 image in the horizontal plane was at most one third of that, 

 which must result froin the inclination given to the optic 

 axes in this direction ; while in the vertical plane the sepa- 

 ration of the images and the inclination of the axes were 

 pretty exactly equal : whence I inferred, that the extent of 

 the tield of swingle vision was three times as great in t';e ho- 

 rizontal plane as in the vertical. But as I have found,, that 

 the inclination of the optic axes in the horizontal plane must 

 be about 15°, to give rise to a double linage, the distance 

 from the centre of the aperture of the pupil, vvhere the 

 rays decussate each other, to the bottom of the eye, on 

 which they are depicted, being about 1'2 mil. [4'721 lines 

 Eng.] in an adult; I tind, that the opening of this angle 

 must comprise horizontally an extent of 8 mil. [3'147 lines 

 Eng.] on the retina: and a third of this extent being taken 

 for the limit of the field of single vision in the vertical 

 planCj this field will be represented by an ellipsis, the longer 

 axis of which is 3' 14? lines, and the shorter r049 line* This 

 however I give but as an approximation. 



These inquiries concerning the field of single vision have Perfection of 

 led me to some reflections on the perfection of the achro- 'he achromatic 



,.1 1 • . T • , . 1 . . , , power of the 



niatic system of the eye, which I shall add here, though eye. 



they are merely accessaries to my principal subject. It is 

 well known, that Euler was led to the discovery of achro- Euler.; 

 matic glasses by considering the structure of the eye. An 

 optical instrument composed of refractive substances of 

 considerable curvature, which receive the luminous rays on 

 great part of their surface, yet form at their tocas an image 

 perfectly well defined, thougli the distance of the object 

 and direction of the rays may be" infinitely varied, exhibits 

 an effect so different from that of ordinary instruments, that 

 Vol. XXV.— Fe». 1810, K ^ this 



