18 REPORT— 1856. 



operation which could be effected in a few seconds by a suitable mechanism. The 

 electrometer would immediately indicate an inductive electrification simply propor- 

 tional to the atmospheric potential at the position occupied by the centre of the ball, 

 and would continue to indicate at each instant the actual atmospheric potential, 

 however variable, as long as no sensible electrification or diselectrification has taken 

 place through imperfect insulation or convection by particles of dust Or currents of 

 air (probably for a quarter or a half of an hour, when care is taken to keep the 

 insulation in good order). This might be the best form of apparatus for making 

 observations in the presence of thunder-clouds. But I think the best possible plan 

 in most respects, if it turns out to be practicable, of which I can have little doubt, 

 will be to use, instead of the ordinary fixed insulated conductor with a point, a fixed 

 conductor of similar form, but hollow, and containing within itself an apparatus for 

 making hydrogen, and blowing small soap-bubbles of that gas from a fine tube ter- 

 minating as nearly as may be in a point, at a height of a few yards in the air. With 

 this arrangement the insulation would only need to be good enough to make the loss 

 of a charge by conduction very slow in comparison with convective loss by the 

 bubbles ; so that it would be easy to secure against any sensible error from defective 

 insulation. If 100 or 200 bubbles, each tV inch in diameter, are blown from the 

 top of the conductor per minute, the electrical potential in its interior will very rapidly, 

 follow variations of the atmospheric potential, and would be at any instant the same 

 as the mean for the atmosphere during some period of a few minutes preceding. 

 The action of a simple point is (as, I suppose, is generally admitted) essentially 

 unsatisfactory, and as nearly as possible nugatory in its results. I am not aware 

 how flame has been found to succeed, but I should think not well in the circumstances 

 of atmospheric observations, in which it is essentially closed in a lantern; and I 

 cannot see on any theoretical ground how its action in these circumstances can be 

 perfect, like that of the soap-bubbles. I intend to make a trial of the practicability 

 of blowing the bubbles ; and if it proves satisfactory, there cannot be a doubt of the 

 availability of the system for atmospheric observations." 



[Addition, Feb. 1857.] — The author has now made various trials on the last-men- 

 tioned part of his proposal, and he has not succeeded in finding any practicable self- 

 regulating apparatus for blowing bubbles and detaching them one by one from the 

 tube. He has seen reason to doubt whether it will be possible to get bubbles so small 

 as those proposed above, to rise at all ; but he has not been led to believe that, if it is 

 thought worth while to try, it will be found impracticable to construct a self-acting 

 apparatus which will regularly blow and discharge separately, bubbles of considerably 

 larger diameter, and so to secure the advantages mentioned, although with a pro- 

 portionately larger consumption of the gas. 



On the other hand, he finds that, by the aid of an extremely sensitive electrometer 

 which he has recently constructed, he will be able, in all probability with great ease 

 and at very small cost, to bring into practice the first of his two plans, constructed 

 on a considerably smaller scale as regards height than proposed in the preceding 

 statement. 



On Printing Photographs, ivith suggestions for introducing Clouds and Artistic 

 Effects. By E. Vivian, M.A. 



The object of this paper was to point out the deficiencies in the chiaroscuro of 

 photographic pictures, occasioned by the discrepancy between the actinic and the 

 visual ray, and also the importance of introducing artistic effects in accordance with 

 the laws of composition. 



The former of these is well known, yellow being the focus of light in the scale oi 

 colour, whilst it is the darkest in the photographic image, the greatest intensity of 

 chemical action in the spectrum being in the violet, and even beyond the range of light. 

 The defects of composition in ordineiry nature are not so generally admitted, but, to 

 the artist's eye, few scenes are capable of producing a good picture, without, at 

 least, the concentration and balance of light and shadow, which are only seen under 

 rare and peculiarly favourable circumstances. Attention to this latter point is the 

 more necessary in most photographic pictures from their reduced size, which 

 requires them to be viewed at a distance from the eye, much beyond the technical 



