Mr. H. Collen on the Application of Photography. 75 



the paper is placed so as to be in the exact focus for obtaining 

 an image of the same size as, or larger than, the object. 

 When the electrometer is the instrument to be registered, the 

 figures of the extreme ends only of the straws are allowed to 

 fall upon the paper, an opake diaphragm pierced with a slit, 

 the curve of which is part of a circle of which the length of 

 the straws is the radius, being placed very near the paper. 



In the registration of the thermometer or barometer, the 

 difficulty arising from the refraction of light by the glass tube 

 was proposed to be met in two ways, the first of which, the 

 one adopted, consists in the use of a diaphragm with a straight 

 slit, which can be opened from, or contracted towards, its exact 

 centre by a very simple arrangement, and is placed in front of 

 the mercury, i. e. on the side next to the light, so as to regu- 

 late the quantity admitted; this regulation has also the effect 

 of preserving the necessary sharpness of figure, which too 

 much light tends to injure. 



The second method proposed, which has not yet been tried, 

 consists in the employment of a piece of glass tube, the bore 

 of which is a trifle larger than the outside of the tube of the 

 instrument; this, having two opposite surfaces ground flat and 

 polished, and being long enough to include the range of va- 

 riation, is cemented on to the tube of the instrument with Ca- 

 nada balsam, and would render it easy (by making all but a 

 central slit opake) to get rid of the partial illumination of the 

 column of mercury on the side which is required, for a good 

 impression on the paper, to be quite dark. 



The surface of the mercury in the barometer sustains a 

 blackened pith- ball of the same diameter as the bore of the 

 tube, but freely sliding therein; it is proposed however to 

 make a float of platinum foil with a sharp edge, which will 

 probably be found to be more advantageous. 



The thermometer used is mercurial, with a broad flat bore. 



The wet- bulb, hair hygrometer, &c, as well as every other 

 instrument which by its action affords a distinct sign, may 

 obviously be registered in the same manner. 



Several minor points of difficulty remain still to be over- 

 come, but it is hoped that enough has been done to justify the 

 expectation that the photographic camera may become a really 

 useful and convenient instrument in the hands of the exact 

 meteorologist. 



The electrical experiments were made by means of a small 

 conductor, insulated for the occasion ; Mr. Ronalds not feeling 

 either authorised, or disposed, to interrupt the course of obser- 

 vations carried on by means of the ordinary high conductor, 

 until the proposed mode of registration is quite matured. 



G2 



