238 Capt. Kater's Appendix to further Experiments 
carefully measured. At the conclusion of these observations, 
the card next the mirror was moved till a perfect image of 
the candle was formed on it, when its distance from the card 
beyond the focus was measured, and the mean of a number of 
these last observations (differing but little from each other) 
was taken, in order to ensure a more accurate result. The^ 
distance of each card from the focus when the light was of 
equal intensity being thus obtained, the squares of those dis- 
tances will express the relative intensity of illumination at equal 
distances on either side of the focus. 
With white circles on black cards. 
Distance of the 
Distance of the 
Ratio of the intensity 
of the liaht at eaual 
white circleswhen 
white circle from 
Distance of the 
distances 
from the 
equally illumina- 
the focus with- 
white circle from 
focus. 
ted. 
out. 
the focus within. 
Within. 
Without. 
Myself 
25,25 
10, 
15.25 
1000 
430 
25.75 
10, 
15,75 
1000 
403 
25,30 
10, 
15,30 
1000 
427 
K 
25,13 
10, 
15,13 
1000 
437 
26,25 
10, 
1&.35 
1000 
379 
25,25 
10, 
15.25 
1000 
430 
C 
24,25 
10, 
14,25 
1000 
492 
25.37 
10, 
15,37 
1000 
423 
24.75 
10, 
14,75 
1000 
460 
Mean 
1000 
431,2 
Conceiving that some error might possibly arise from the 
one circle being nearer to the eye than the other, I substituted 
for them transparent disks of six-tenths of an inch diameter. 
