on the Light of the Cassegrainian Telescope, &c, 239 
n:ade by cutting circular holes in two slips of black card paper, 
and covering them with fine oiled paper. Small shades of black 
card paper were contrived to shield the disks from the direct 
rays of the candle. The disk within the focus being made the 
brightest, was moved at intervals towards the mirror till the 
observer pronounced both disks' to be equally illuminated, 
when their distance was registered as the first reading ” The 
disk within the focus was then advanced so near the mirror, 
as to appear much the faintest, and afterwards brought back 
gradually till the iliuminarion of the disks was a second time 
pronounced to be equal, when their distance was again mea- 
sured and entered under the head of “ second reading** and 
the same method was followed in all the subsequent experi- 
ments. 
With large transparent disks. 
Distance of the candle from the mirror 186 inches. 
Observers. 
Distance of the disks 
when equally illumi- 
nated. 
Distance of 
the disk Irom 
ihe focus 
without. 
Distance of 
the disk from 
the focus 
within. 
Ratio of the inten- 
sity of the light at 
equal distances from 
the focus. 
IS! reading. 
2d reading 
Within. 
Without. 
iVIyself 
K 
2645 
24. 5 
25,70 
3 5 > (>' 
3 
10,22 
15.77 
1 4,83 
1000 
1000 
427 
475 
Mean 
1000 
45 > 
In making the third set of experiments I remarked that a 
part of the light from the mirror was received on that portion 
of the card which was blackened, and fearing that this circum- 
