302 Mr. B. S. Proctor on the Focal Adjustment of the Eye. 



Fatigue diminishes the power of the eyes to obtain the extremes 

 of long or short focus, and decreases their resolving power at 

 all distances. Some observers find that a focus which cannot 

 be immediately obtained will come after a little perseverance ; 

 others find that a sudden effort is most successful ; in both cases, 

 when the focus has been obtained only by an effort, it cannot 

 be long retained. Thus on the Table, the shortest focus for 

 vertical lines which could be obtained by T. P. B. is given as 11 

 or 12 inches; but that was obtained only by an effort, and the 

 shortest focus, for the same lines, which could be retained suit- 

 ably for examining the resolving power, was found to be about 

 2 feet. 



When the length of the ocular focus varies, the relation between 

 the horizontal and vertical foci do not Change, that which is long- 

 est at one time being always longest. Some observers say they 

 cannot do fine work before breakfast ; others say they can read 

 before breakfast, with the naked eye, type which requires specta- 

 cles at any other time of the day ; some others, myself included, 

 have failed to find any difference, with carefully tried experi- 

 ments. 



I have used the words " resolving power " to express a power 

 which varies with several physical circumstances, but it is not 

 very easy to decide which are the usual or principal causes of its 

 variation. It is not unlikely that, as regards the formation of 

 an image, chromatic and spherical aberrations, and aberrations 

 depending upon the curves of the eye being distorted by mus- 

 cular and other causes, are at least the principal causes of its 

 variation. And as regards the perception of the image, the sen- 

 sitiveness of the optic nerve, variation in the degree of its rami- 

 fications, and education are the most important. 



The resolving power is necessarily greatest when the light is 

 moderate ; with the apparatus before described, the naked flame 

 behind the test-object when near the eye was generally found too 

 bright ; greater resolving power was obtained by interposing a 

 piece of oiled paper, and in some cases the interposition of 

 writing-paper without oil was still better ; at a distance of two 

 or three feet the stronger illumination was generally preferred. 

 In like manner, when the test-object was placed at the end of a 

 telescope-tube (an arrangement I used for making observations 

 by daylight), I found that for my own eye, sunlight reflected from 

 white paper was too powerful an illumination, and even sunlight 

 reflected from a carpet of dull colours was less advantageous than 

 diffused daylight reflected from the same. But it is probable 

 that the degree of illumination most advantageous will vary 

 greatly with even the same eye according to the nature of the 

 object, grey lines requiring a stronger light when upon either 



