J. W. Draper—Distribution of Heat in the Spectrum. 171 
covered the upper and lower portions of the concave mirror dd 
with pieces of black paper, so arranged as to leave a band of 
sufficient width to receive and reflect the entire spectrum. 
aa, fig. 4, is the upper paper, bb the lower, ce the uncovered 
reflecting band, receiving the spectrum 
rv. Had the spaces thus covered been 
permitted to reflect, they would have 
rendered more intense the image of the 
Fig. 4. 
could not be used in these delicate re- 
: searches until proper arrangements were 
applied. It was covered with a glass shade. The slightest cause 
casioned currents in its included air, which perpetually drifted 
and disturbed the needles. For this reason, and also 
accurate reading, it is best to view the position of the index 
tough a small telescope. 
e combination of needles being nearly astatic, attention 
Must be paid to their magnetic perturbations, whether arising 
from local or other causes; and, since the vibrations are very 
i. ample time must be given before the reading is ascer- 
ed. 
The condition of the face of the pile is of importance. It 
Wil not answer—the surface so produced is too glossy and 
rellecting. The plan I have found best is to take a glass tube 
If an inch in diameter and six inches long, open at both ends. 
and use it as a chimney. Api 
the lower end, and the face of the pile to be blackened being held 
Stanc 
falling on it, Its quality of transmitting light is well known to 
every one who has looked at the sun through a smoked glass. 
e galvanometer I have used is calibrated according to the 
ethod. e num given in this memoir do not 
Ped the angles of deflection, but their corresponding 
