112 MR. RONALDS ON PHOTOGRAPHIC SELF-REGISTERING INSTRUMENTS. 
projecting a strong image of the electrometer, in deep oscuro, upon it. Through the 
screen a very narrow slit, of proper curvature, is cut (the chord of the arc being in 
a horizontal position), and it is fitted into the back of a case, about two and a half 
feet long, which case is fixed to the eye-end of the microscope, at right angles with 
its axis, and vertically. Within this case is suspended a frame, provided with grooves, 
into which two plates of pure thin glass can be dropped, and brought into close con- 
tact by means of six little bolts and nuts. This frame can be removed at pleasure from 
a line, by which it is suspended, and the line after passing through a small aperture 
(stopped with grease) cut through the upper end of the long case, is attached to a 
pulley (about 4 inches in diameter) fixed, with capacity of adjustment, on the hour- 
arbor of a good clock. Lastly, counterpoises, rollers, springs, and a straight ruler 
are employed, for ensuring accurate rectilineal sliding of the frame when the clock 
is set in motion. 
A piece of properly prepared photographic paper is now placed between the two 
plates of glass in the moveable frame ; the frame is removed (in a box made purposely, 
for excluding light) and is suspended in the long case ; this is closed so as to prevent 
the possibility of extraneous light entering it; the clock is started; and the time of 
starting is noted. 
All that part of the paper which is made to pass over the slit in the screen, by the 
motion of the clock, becomes now therefore successively exposed to a strong light ; 
and is consequently brought into a state which fits it to receive a dark colour on 
being again washed with the usual solutions, excepting those small portions upon 
which dark images of the lower parts of the pendulums of the electrometer are pro- 
jected through the slit ; these small portions of course retain the light colour of the 
paper ; and form the long curved lines or bands, whose distances from each other 
at any given part of the photograph (i. e.), at any given time, indicate the electric 
tension of the atmosphere at that time. 
Sometimes, when daylight was used, various appearances of the sky were noted 
during the process, by which it would seem, that, in serene weather, when the sun’s 
light and heat varied, and the paper became consequently more or less darkened, 
the electric tension, as shown in the photograph, varied also; increasing with the 
increment of light, &c. This fact has not perhaps been before observed : and some 
attentive observations on the subject, made with the aid of a good actinometer, &c., 
are desirable. 
In order that the state of the electrometer itself may be known at any period of 
the process, a small microscope is fitted to an aperture in the door of the long case, 
opposite to the slit in the screen, and arrangements are made whereby the eye may 
be applied to it, and to view the images through the semitransparent photographic 
paper, without damage by the admission of extraneous light. 
The adjustment of the lenses in the body of the lucernal microscope, for procuring 
the best possible chemical focus, can only be obtained by a short series of experiments; 
but having been once found, future adjustments are not necessary. 
