116 MR. RONALDS ON PHOTOGRAPHIC SELF-REGISTERING INSTRUMENTS. 
passing through a large aperture in the bottom of A. b 7 . A pith-ball 
fixed on b 5 . b s , b 8 . Two small Leyden jars, of very thin glass, fitted 
upon sliders, which can be made to approach or recede from each other 
by means of adjusting screws, b 9 , b 9 . Wires connected with their 
interior coatings, and capable of adjustment for height, b 10 , b 10 . Cross 
wires, each carrying at one end a brass ball, and at the other a small 
electroscope, of straws. 
When this distinguishing electrometer is in use, the jars are charged 
artificially, one negatively, the other positively : consequently, if B 
receives a positive charge from the atmospheric conductor, b 7 is at- 
tracted toward the negative ball, and b 6 inclines in that direction, and 
vice versa. 
It is evident, therefore, that in the resulting photograph a line must 
appear between the two lines, produced by the images of the short 
straws b l , b x , but nearer to one than to the other. In order to ascer- 
tain with certainty, in low intensities, to which ball it inclines, a 
very small bar (e 2 , fig. 7) is placed across the exact centre of the slit in 
the screen ; the shadow of which bar (e 2 ) of course creates a central 
line in the photograph. 
These jars retain a sufficient low charge for twenty-four hours even 
in a humid state of the air (for they are somewhat similarly circum- 
stanced to an electrophorus, which will frequently retain a low charge 
for one or two months). 
Fig. 7- The screen E (vide fig. 1) with its slit e l , the visible parts of the images of the 
short straws b l , b l , that of the long straw b 6 , and the bisecting little 
cross-bar e 2 . 
PLATE XL 
Fig. 8. The Declination Magnetograph. 
AV. A box divided into two compartments. 
W. An interior box. 
A. The lucernal microscope. 
B. The declination magnet, b 2 . Its stirrup, from which it may be removed 
at pleasure, and the usual brass bar substituted after relaxing two 
milled-headed screws and turning downward the nearest sides of the 
stirrup, b 3 . The pair of light tubes connected with the stirrup by means 
of a short tube, which permits a horizontal adjustment of b 3 . The 
counterbalance on the nearer end of b 3 is also adjustible. 
b l . The index, b 4 . The damper, the central parts of which, above 
and below, are expanded and form rings. 
