As A. C. Twining on Occultations by the Moon and Planets. 23 
the rectifications were well complete. Under the most favora- 
ble circumstances this species of observation is liable to large — 
iserepancies,—however, a mean of many observations may give 
a reliable average. The observations eee at each graduated 
distance were limited to four pairs, In the aati table ea 
first, or left-hand, column contains the number 0: observ: 
tion or eye’station,—the second column gives in Ghehes the Pod 
tances of the eye at those stations from the paper,—the four 
columns following record the corresponding lamp distances, as 
apa The seventh column contains averages of the four 
ts of observations,—and the last is a column of empirical dis- 
a computed for comparison, after the law of a common 
difference septs by those ow At the nearest station 
tinct conception of the visual object for a visual effect. 
Tabular observations, April, 1847. 
pois 4 i Lamp distances. _ \|Averages age oe 
1 } “11 14°5 14° 145 155 148 
“9 “| yor29 34°38 | 29°5 27-6 264 29°5 30°8 
. 2 I 85-88 40°5 517 46°5 473 46 
Pete 68°66 5T4 692 619 64-7 63°3 
5 54°93 44:9 {71 q47 791 765 188 
6 43°95 99 905 883 90-2 94-8 
4 | 35°16 Vil 1065 1100 1114 | 1106 1108 
8 9e19 | 1984 | 1996 | 13821 | 1884 | 1316 {4 : 
It will be seen that this table, notwithstandin discrepancies = 
—- from imperfect ee ee pe a y indicates the fol- 
rema. nkable result of this law is noe small es onal 
faint light possesses as great efficacy to balance a 
ng eft asa large fractional change of a much 
righter Ii nthe 
antnien unity, we have the illumination at the second sta- 
tion less than 4. Here thefetbre over $ decrease of the light is 
required to balance a magnifying effect of ¢. But, in the next 
A from the second to the third, the same power 5 is balanced 
a decrease of only about sths of the light and, in the 
fom the last but one to the last, the same {ths of power is ot 
