THE SENSE OF SIGHT 693 



small interval between the two images on the retina of the two objective 

 points. The size of the image or the interval between the two retinal points, 

 determined from the foregoing factors by the formulae on page 692 is 0.004 

 mm., which would correspond to a visual angle of 60 seconds. If the 

 retinal distance is less than this the two sensations fuse into one. The reason 

 assigned for this is, that the distance between the centers of two adjoining 

 cones in the macula is 0.004 mm. With a visual angle not less than 60 

 seconds, the two foci fall on separate cones. With a smaller visual angle 

 the two foci fall on, and excite but a single cone and hence there arises the 

 sensation of but a single point. The acuteness of vision, therefore, of the 

 emmetropic eye depends on its power of distinguishing the smallest retinal 

 image or the smallest interval between two cones on the retina, correspond- 

 ing to a visual angle of 60 seconds. 



In ophthalmic practice it is customary in testing the acuteness of vision 

 to employ test letters of specific sizes for specific distances. The letters are 

 so proportioned that when they are placed at the specified distances, the 

 extremities of the letters subtend an angle of 5 minutes. The letters have 

 been constructed on the following basis: Since to an angle of 60 seconds 

 there corresponds an object of 1.454 mm. at the distance of 5 meters as 

 shown before and as the object decreases in proportion to the distance 



L 



FIG. 300. STANDARD LETTERS, FOR TESTING THE ACUITY OF VISION. 



(for the same visual angle) it is evident that the object would have to be one- 

 fifth of i 454 mm. or 0.2908 mm. in order to subtend an angle of 60 seconds 

 at one meter. From this the size for any other distance in meters is found 

 simply by multiplying 0.2908 mm. by the distance. The standard letters 

 are so constructed that each is inscribed within a square, the sides of which 

 at a specific distance, 5 meters, subtend an angle of 5 minutes and which is 

 again subdivided into 25 small squares each side of which subtends an angle 

 of i minute. These partial little squares correspond to the details of the 

 letter while the whole letter of course, embraces an angle of 5 minutes both 

 as to height and to breadth (Fig. 300). The letter that could be distinctly 

 seen at a distance of 5 meters, would have, therefore, a vertical and a hori- 

 zontal dimension of 5 times i. 4 54 mm. or 7.27 mm. (Fig. 300 A), and 

 meters corresponding dimension of 14-54 mm., etc. (Fig. 300^.) 



If with the accommodation suspended, the emmetropic eye could 

 clearly distinguish at a distance of 5 meters a letter 7.27 mm. m size which 

 would, therefore, subtend an angle of 5 minutes, then the acuity of the 

 vision would be normal and could be expressed as follows: V =^or V -T* 

 If on the contrary at this distance the smallest letter that could be clearly 

 seen is one that would subtend an angle of 5 minutes at a distance of 10 meters 

 then the visual acuity would be only one-half the normal, and could be 

 expressed as follows: V = A or V - J, etc. The acuity of vision is expressed, 



