TRANSACTIONS OP THE SECTIONS. 19 



Regulateur Automatique de Lumiere Electrique. By M. Serrin. 

 To form the electric arch of light, it is first necessary to bring the charcoal points 

 into contact, then gently to separate them by degrees, as they glow, afterwards to 

 cause them to approach constantly, as they are wasted by use, carefully avoiding 

 bringing them into contact. In order to keep the point of illumination fixed in space, 

 each charcoal point must simultaneously approach the other, and that in the pro- 

 portion in which each is wasted by use. In fine, for rendering the electric light 

 useful, all these conditions must be self-produced with the utmost regularity, with- 

 out any intervention of the human hand, that is to say, in a manner completely 

 automatic ; and this was the object the regulator was invented for. In a simple 

 and easy manner, this apparatus, which may be compared to an extremely sensible 

 balance, is composed of two mechanisms connected the one with the other, and yet 

 independent ; when one acts the other is in repose, and reciprocally. One of these 

 consists of an oscillating system, — the chief feature of the regulator destined to pro- 

 duce the separation of the charcoal points, and also to determine their re-approach. 

 The other mechanism, composed of wheel-work, has for its object to ensure the re- 

 approach of the charcoal points in the proportion of their waste by use. The two 

 port-carbons which carry the charcoal pieces are placed vertically one above the 

 other. The superior is in connexion with the wheel-work, and is the positive elec- 

 trode of the battery; the inferior depends as well on the wheel-work as on the 

 oscillating system, and is the negative electrode. The superior port-carbon, by its 

 weight, causes the inferior to ascend. The oscillating system forms a parallelogram, 

 of which the angles are jointed, one of the vertical sides of which is suspended by a 

 spring, and carries at its lower part a soft iron armature, placed over a horizontal 

 electro-magnet. When the apparatus is in repose, the charcoals are in contact ; on 

 the contrary, they separate when the circuit is completed and the voltaic arc ap- 

 pears. As the wasting by use of the charcoals increases the length of the voltaic arc, 

 the armature increases its distance from the electro-magnet, become less powerful, 

 and the charcoals re-approach by a quantity frequently less than the one-hundredth 

 of a millimetre ; but according as they re-approach, the electro-magnet recovers its 

 original power, the armature is attracted anew, and the charcoals stop until a new 

 wasting gives rise to a new re-approach followed by a new stoppage, and so on in 

 succession. In consequence of its extreme sensibility, it will work either with a 

 voltaic pile or an electro-magnetic machine. 



On some Recent Extensions o/Prevost's Theory of Exchanges. 

 By Balfour Stewart, M.A. 



On Rings seen in viewing a Light through Fibrous Specimens of Calc- 

 spar. By G. Johnstone Stoney, M.A., F.R.A.S. fyc. 



The author mentioned that Sir David Brewster had drawn the attention of the 

 Association, at the York meeting, to the beautiful display of four rings which may 

 be seen on looking at a luminous point through fibrous specimens of calc-spar. 



In the present communication the forms of the rings were traced as a consequence 

 of Huygen's construction, and the points where rings vanish, or where irises pass 

 into one another, were determined. The state of polarization was also examined, and 

 the positions, in which two of the rings, which are faint, will be most conspicuous. 

 The author drew particular attention to the great range of brightness of these faint 

 rings, and to the circumstances attending the disappearance of one of them, in con- 

 sequence of a curious case of impossible reflexion, as offering peculiar facilities for 

 testing rival hypotheses. 



On Thin Films of Decomposed Glass found near Oxford. 

 By R. Thomas. 

 The films were observed on bottles of the form called magnums, which had been 

 lying in the Cherwell above a century. The films formed by decomposition on the 



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