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X. On Photographic Self-registering Meteorological and Magnetical Instruments. 
By Francis Ronalds, Esq., F.R.S. 
Received November 18, 1846, — Read January 21, 1847. 
The Council of the Royal Society having-, in April last, granted to me, out of the 
Donation Fund, fifty pounds “ for the purchase of magnetical and meteorological 
instruments” necessary to the prosecution of experiments in which I was then (and 
had been long previously) engaged, I feel very anxious to express, now, my deep 
sense of obligation for its munificence, and to endeavour to show that successful and 
satisfactory results, in great measure due to the timely assistance thus kindly afforded, 
have been attained. 
It would be superfluous to speak of those proposals of other gentlemen, or of my 
own, for self-registering, photographically, the variations of the declination magnet 
and the thermometer which were made previously to the use of good achromatic lenses, 
for projecting, upon photographic paper, a sharp image , magnified to any required 
degree, of that part of the instrument whose motions are to he registered. 
This is the principal feature of the system which I have hitherto applied to procure 
the self-registration of the atmospheric electrometer, the thermometer, the barometer, 
and the declination magnetometer, and which I propose to apply to every other me- 
teorological and terrestro-magnetical instrument. Although it had long occupied 
my thoughts, had received some approbation from the Astronomer Royal in April 
1845, and had been the object in view in some experiments made in conjunction with 
Mr. H. Collen of Somerset Street, on photographic paper in the beginning of July 
1845, yet my other contrivances and occupations at the Kew Observatory (elsewhere 
detailed) prevented the completion of any apparatus for actual registration until the 
end of that month. 
The photo-electrograph as then constructed, and since improved, may be thus de- 
scribed. 
A rectangular box, about 16 inches long and 3 inches 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 this 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 con- 
densing lenses, when a common Argand lamp is employed. In either case an abun- 
dance of light is thrown into the microscope. Between the electrometer and the other, 
or eye-end of the microscope, fine achromatic lenses are placed, which have the double 
effect of condensing the light upon a small screen, situated at that eye-end, and of 
