662 



of a slip of gold leaf. Thus it appears that the substance which 

 possesses the highest chemical affinity manifests also the greatest 

 power of electrical tension. 



. January 21, 1847. 

 The MARQUIS OF NORTHAMPTON, President, in the Chair. 

 Henry Dyke Acland, M.D. was elected a Fellow of the Society. 



" On Photographic Self-registering Meteorological and Magneti- 

 cal Instruments." By Francis Ronalds, Esq., F.R.S., &c. 



The apparatus employed by the author at the Kew Observatory, 

 and which he terms the Photo- Electrograph, is described by him 

 in the following words : — " A rectangular box, about sixteen inches 

 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 achromatic 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 he 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. 



