128 Royal Society. 



long and three square, constitutes the part usually called the body of 

 a kind of lucernal microscope. A voltaic electrometer (properly 

 insulated, and in communication with an atmospheric conductor) is 

 suspended within the microscope, through an aperture in the upper 

 side, and near to the object end. That end itself is closed by a plane 

 of glass, when daylight is used, and by condensing lenses, when a 

 common Argand lamp is employed. In either case an abundance 

 of light is thrown into the microscope. Between the electrometer 

 and the ether, or eye-end of the microscope, fine aehromatic lenses 

 are placed, which have the double effect of condensing the light 

 upon a little screen, situated at that eye-end, and of 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 the case is suspended a frame, provided with a 

 rabbet, into which two plates of pure thin glass can be dropped, and 

 brought into close contact 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 four 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 possi- 

 bility of extraneous light entering with 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 suc- 

 cessively 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 solution, excepting those small portions upon which 

 dark images of the lower parts of the pendulums of the electrometer 

 are projected through the slit. These small portions of course re- 

 tain the light colour of the paper ; and from the long curved lines 

 or bands, whose distances form 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. 



" By certain additions to the instrument above described, the kind 

 as well as the tension of electrical charge is capable of being re- 

 gistered ; and by the employment also of a horizontal thermometer, 

 &c., it is adapted to the purposes of a Thermograph, as well as 

 Photo-baromctrograph and Magnctograph." 



