EEPORT OF THE ANTHROPOMETRIC COMMITTEE. 263 



occapations, involving tho employment of the sight in a similar manner 

 miwht be tested, and conld afford materials for very important con- 

 clusions. 



Note. — The conditions under which the acuteness of vision is tested 

 have a marked influence on the results. The most favourable circum- 

 stances for bringing out the acuteness of vision in persons whose eyes 

 are healthy, and their refracting power normal, are to view a dark body 

 against a well-illuminated bright- coloured background some way behind 

 it. For short distances the upright or wire of an iron fence ; for greater 

 distances a pole or flagstaff projected against the clear sky beyond, are 

 suitable objects, from either of which the distance of tbe observer can 

 be measured. The writer, for instance, though his vision is no longer 

 so acute as formerly, can perceive distinctly at 200 yards' distance, in 

 good daylight, the upright of an iron hurdle presenting a breadth of 

 0'36 inch from his point of observation, and projected against a newly 

 gravelled walk ; and, some years ago, he had frequent opportunities of 

 observing a flagstaff, on a hill, the smallest part of which was not more 

 than six inches in diameter, yet in favourable weather that was distinctly 

 visible three miles off. The angle the object subtended at the eye in the 

 fonner instance is 10"'3, and in the latter Q"'7, and in each case it was 

 considerably within the extreme limit of vision. Tt will be observed, 

 however, that though these objects were narrow, they were of consider- 

 able length, and this extension in a direction perpendicular to their 

 smallest diameter enabled the eye to seize and retain the latter, while, 

 had its length not exceeded its breadth there would have been far more 

 difiiculty in finding it in the first instance, and in keeping the eye fixed 

 steadily on it afterwards. In practice objects which appear square or 

 circular, as presented to the eye, cannot be distinguished under such 

 small angles, and only become distinctly visible at distances proportion- 

 ately much shorter. When they are dark, on a light ground in the same 

 plane, the range of vision is reduced still more. 



In arranging his test-types Snellen proceeded on the assumption 

 that the normal eye could perceive an object subtending an angle of 1', 

 and consequently he gave the lines forming his letters such a breadth 

 that they should reach the eye under an angle of 1', when held at the 

 distance indicated in the accompanying instructions ; but to admit of 

 the various parts of the letters being perceptible their height was fixed 

 at 5', or five times the breadth of their limbs. These letters afford a very 

 convenient practical test of the various alterations the sight may have 

 undergone ; but, as will appear immediately, owing to their forms and 

 the small distance between their different parts, they are not adapted 

 for determining quantitatively its greatest range under normal conditions. 

 The test-dots which were introduced originally to ascertain the fitness of 

 recruits for military service seem better suited for this purpose, though, 

 as they were employed to ensure the minimum range of vision required 

 for military service, which is far within that of the ordinary healthy eye, 

 their capabilities were never fully considered or developed. 



A single test- dot, say a square of ^ of an inch of a side, will be 

 visible as far as any combination of them in which the single dots are 

 distinguishable ; their arrangement in groups is merely to provide a 

 check on the person under trial, by varying the number exposed and 

 requiring him to count them ; but when investigating the extreme limit 

 of vision a difficulty arises here, which did not pi'esent itself when 



