38 WILD TRIBES OF PERAK. 
Semangs.—Only three of these people have been tested as yet. 
The figures are as follows :— 
Distance at which 
Semang of Dex, Age. test-spots were 
counted. 
Upper Perak me ie ...| Male 14 92 feet 
a ae ... | Female 20 635i 
* Male 40 SOs ny; 
The range is here from 50 to g2 feet, and the mean 68°33 feet. 
The one woman is practically the same, as regards sight, as the mean of 
the Sakai women. 
The theoretical limit of resolving power of the human eye has been 
placed at objects which subtend a visual angle of 25°8 seconds. Lord 
Rayleigh, writing under the heading Cvvilisation and Eyesight, says, 
“It is known to physicists that the resolving power of an optical instru- 
ment is limited by its aperture. With a given aperture no perfection of 
execution will carry the power to resolve double stars, or stripes alter- 
nately dark and bright beyond a certain point, calculable by the laws of 
optics from the wave-lengths of light.”” Mr. Sydney Lupton, writing on 
the same subject, says, “In the case of a small angle the aperture 
divided by the distance is approximately equal to the arc divided by the. 
radius or to the circular measure of the angle. Hence in the present 
-1/40,o0oth inch I 206,265 
1/5th inch e 8,000 8,000 = 258 seconds 
radian, or 
case we have 
nearly.” 
According to this, the utmost range at which the army test-spots, 
of one-fith inch diameter, could be resolved by the unaided human eye 
15.133 fect, 
= 133'24 feet, the length of the radius of a circle, 
206,264'°8 + 25°8 
( 120 + 2 
25°8 seconds of which would measure one-fifth of an inch). 
In the statistics above given the greatest distance at which the test- 
spots were read was 92 feet. This would equal an angle of 37°35 seconds. 
( 206,264'8 
1/60 foot x g2 feet = 37350 seconds.) 
The mean of the 29 Sakai men, that is 58°39 feet, gives an angle of 
58°87 seconds. 
The common test for eyesight in England is the greatest distance at 
which ‘diamond’ type can be read; and Mr. Brudenell Carter says 
“the commonly accepted standard of normal vision is satisfied by 
deciphering letters the parts of which subtend visual angles of one 
minute’’—that is, about one and a quarter seconds more than that found 
for the Sakais, 
