on the Light of the Cassegrainian Telescope^ &c, 237 
Observers. 
Diameter of the illu- 
minated civclewithhi 
the focus. 
Diameter of the cir- 
cle of equal illumi- 
nation without the 
focus. 
Ratio of the intensity 
of the light at equal 
distances from the 
focus. 
Within. 
Without. 
Y 
24,3 
15.75 
1000 
430 
Y 
24^3 
16,00 
1000 
433 
K 
24’3 
14^50 
1000 
352 
K 
24^3 
i6,s!5 
1000 
447 
K 
24^3 
16,25 . 
1000 
447 
M 
24^3 
16,00 
1000 
433 
M 
24,3 
16,50 
1000 
46*1 
M 
24^3 
15,00 
1000 
381 
Mean 
1000 
421,6 
Not being perfectly satisfied with the manner in which these 
experiments were conducted, I was anxious, to devise some 
means of viewing the light on both sides of the focus at the 
same time; and this to a certain degree I accomplished in the 
following manner. 
I caused two small blocks of wood to be made to slide easily 
on the rod beforementioned, and cut in each a notch with a 
fine saw to receive a slip of card at right angles to the rod. 
On each card a circle of half an inch in diameter was drawn 
which remained white, the rest of the card being painted black. 
By this contrivance, I could receive on each circle a portion of 
the light reflected from the mirror when the slips of card were 
placed one within the focus, and the other without. The one 
without the focus remained stationary, whilst that within was 
shifted till the observer pronounced the white circles to be 
equally illuminated, when the distance between the cards was 
