148 0. N. Rood on Photometric Experiments. 
glass, G, finely ground on each side; this is destined to receive 
the light from L. If only one surface of the glass be ground, the 
texture becomes plainly visible, particularly when, as in the ex- 
periments detailed in the second part of this paper, the spot is 
magnified. Only so much of the ground glass plate is allowed 
to remain bare as is necessary, the remaining portions on both 
sides are covered by black non-reflecting paper. Plates P and 
G are then as it were roofed over, and enclosed on all sides by 
Source of ‘llumination.—At E is the eye of the observer, the 
face being protected from light by the blackened screen, 8. 
the center of the “spot,” and the center of the mirror expert 
mented on, all lie in the same line, which is of course at the 
same time the axis of the instrument. At H are two small gas 
flames issuing from circular apertures, and destined to illuminate 
the collodion plate on the side next to the observer; both are 
fed from the same source. The gas-burner at H consists of two 
thin brass tubes, half an inch in diameter and one inch long; 
connected together by a glass tube; the circular apertures for 
the flames are placed at equal distances on either side of the 
~ 
t, an from it as is found most advantageous im aly 
particular set of experiments; their distance from eac other 18 
seven inches. et lig m the two flames is pre 
i pe re, 
by small blackened screens, the same means being ee ep 4 
it in i yo om 
gas flame is employed at H, the ground around the spot will > 
un ly illuminated, and exact observations become imposs 
ble. Of course the direct light from these two burners which 
penetrates through the “spot,” must not be allowed to 
hat portion of the ground glass opposite it; the distance of 
the flames apart must be so chosen that this becomes impossible. 
The light from the movable burner on the other side of the 
e 
screen at L will be used direct and reflected. The small single 
movable ole which supplies it, is similar to those al 
or to it is attached a flexible india-rubber tube, 
which is supplied from the same source that feeds H; it is com 
column, (for the sake of insulating the heat) 
ei 
Dict emi, 
1 
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