ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 247 
standard, by a process quite different from Herschel’s, extending the experiments also 
to the phases between conjunction and quadratures. 
I have used, as a standard light, a little disc of the central part of the flame of a 
Carcel lamp. The height of the flame being first regulated to its maximum of 1".50 
to 17.75 from the top of the wick, a tube of zinc was placed outside of the chimney, 
at a distance of 1 inch from the nearest part of the flame, and high enough to cut off 
all the light excepting that which passed through apertures made in the zinc for the 
purpose; these apertures were circular holes of the following diameters : — 
Diaphragm X 0:094 
« Y 0.056 
“ z 0.040 
« M 0.033 
_ "They were placed in turn opposite to the part of the flame which appeared of most 
uniform brightness. By using, in this way, only a small area of the middle of the 
flame, its variations in height were made much less sensible than they would otherwise 
have been. 
To compare the light transmitted through the aperture with the Moon, the light of 
both objects was received on a glass globe, either silvered or filled with mercury, and 
their images, viewed side by side, were equalized by moving the bulb to the proper dis- 
tance from the lamp. This distance being always large compared with the radius of 
the bulb, the brightness of the two objects will be as the squares of their distances 
from the bulb. 
The following are the diameters of the glass spheres used in these and "MERE 
experiments. 
Diameter of Reflecting Surface. 
B, Filled with mereury (thermometer bulb), S $ i : 0.604 
B, Silvered, . e e ¿ * d š ; j S 8.436 
B, Silvered, ; i i i i j c; 1780 
B, Filled with mercury Ge bulb), e . i : 0.304 
B, Silvered, : ` Í a i > : e i . 10.160 
These refer to the inner reflecting surface of each, twice the thickness of the glass 
having been subtracted from the outside diameters. The spherical form was well pre- 
served throughout the surface of the bulbs, excepting near the stem, a part not used 
in the experiments. 
