on the Light of the Cassegrainian Telescope ^ &c. 241 
Imagining that the superior whiteness of plaister of Paris 
might render a slight difference of light more perceptible than 
it could be by the card disks> I prepared two balls of this sub- 
stance by casting them in a bullet mould, and supported them 
on black wires. With these the following experiments were 
made. 
V . '■ 
With Plaister of Paris balls. 
Distance of the candle from the mirror 186 inches. 
Ratio of the in ten- 
Distance of the balls when 
Distance 
Distance 
sity of the light at 
Observers 
equally illuminated. 
of the balls 
of the balls 
eaual distances trom 
from the 
from the 
the focus. 
focus 
focus 
'wit/juul. 
within. 
1st reading. 
2 d reading. 
mean. 
Within. 
Without. 
w 
24,20 
24»43 
24.31 
10, 
14.31 
1000 
488 
K 
25.5° 
24,8 
25,15 
10, 
15,15 
1000 
436 
Myself 
25,40 
25.67 
25,53 
10, 
15.53 
1000 
415 
W 
H ’77 
25.60 
25,18 
10, 
15,18 
1000 
434 
K 
24.77 
24.15 
24,46 
10 , 
14,46 
1000 
478 
Myself 
24.85 
24.77 
24,81 
10, 
14.81 
1000 
456 
Mean 
1000 
451.1 
Lastly, having made only two experiments with transparent 
disks, and those being of an inconvenient size, I prepared others 
four-tenths of an inch in diameter, which were constructed 
and used as before. 
1 i 
MDCCCXIV. 
