

TRANSACTIONS OP THE SECTIONS. 27 



On a new Optical Instrument. By Sir John Robison. 



Sir John Robison informed the Section that he had lately ascertained, by trial, 

 that on a solid rod of glass being plunged into such a cavity as that of the external 

 car, the light reflected inwards, along the interior of the cylinder, illuminates the 

 bottom of the cavity so as to render it distinctly visible through the matter of the 

 glass, provided it be homogeneous, and the ends of the rod be properly figured and 

 polished. He suggested that instruments on this principle should be made for the 

 examination of such cavities. 



To the Secretaries of the Twelfth Meeting of the Association. 



I offer my warmest and most sincere thanks to you, illustrious and honoured Sirs, 

 for the honour of the invitation which you had the complaisance to send me in the 

 obliging letter of the 13th April, 1842, to attend the Twelfth Meeting of the British 

 Association for the Advancement of Science. I am very sorry to be unable to be 

 present. In order to respond in some measure, and as much as is in my power, to 

 an honour so conferred on me, I have desired my brother, who is at Turin, to send 

 to the Meeting a copy of my Memoirs, published in Modena since 1836, and I have 

 made an Abstract of my Sixth Memoir (not yet published) upon the Magnetizing 

 Action of Transitory Electric Currents, which I take the liberty of sending you here- 

 with. I trust that these productions may not seem unworthy to merit the attention 

 of the meeting. I confide in your kindness and in that of your learned countrymen, 

 and I hope you will receive my communication as a proof of my good wishes, and 

 as a humble testimony of gratitude and of the high esteem I entertain towards the 

 British Association. 



I beg you to accept the sentiments of lively gratitude and profound esteem and 

 consideration with which I am proud to subscribe myself, Gentlemen, 



Your most obedient servant, Stefano Marianini. 



Modena, 1st June, 1842. 



Abstract of an unpublished Memoir upon the Magnetizing Action of Trans- 

 itory Electric Currents, in which it is proposed to explain the Varia- 

 tions in Magnetic Susceptibility which are frequently observed in Iron 

 as often as it has been magnetized. (Addressed to the Twelfth Annital 

 Meeting of the British Association for ilie Advanceme?it of Science.) By 

 Professor Stefano Marianini. 



A piece of iron magnetized to a certain degree, and afterwards deprived gra- 

 dually of its polarity by the circulation of electric currents round it, presents the phie- 

 lomenon, that, by a given current, it is magnetized more strongly or more feebly 

 "lan it is in its natural state, according as the given current is so directed as to pro- 

 duce the north pole in the same part in which it had been produced from the begin- 

 ning, or is directed in the contrary way. This phenomenon furnished, three years 

 ago, the subject of my Second Memoir upon the Magnetizing Action of Transitory 

 Electric Currents. But I then limited myself to the study of the laws of the pha?- 

 nomenon itself, of which it may not be inopportune to transcribe some here, for the 

 better understanding of what follows : — 



1st. If the magnetic susceptibility of a piece of iron be in one direction increased 

 (that is, so that it may have the north pole at a given extremity), and if it be 

 diminished in an opposite direction, the susceptibility may be inverted by treating the 

 same iron with currents or other magnetizing actions contrary to those first employed. 



2ndly. The increase of magnetic susceptibility in one direction equals the decrease 

 of magnetic susceptibility in an opposite direction. 



3rdly. If the action of a given magnetizing current is repeated upon the same 

 iron, the alteration in the susceptibility goes on always diminishing. 



After having long studied the means of depriving the iron of magnetic polarity, 

 and especially the difference which occurs between the de-magnetizings produced by 

 operations which serve also to magnetize, and those produced by operations which 

 do not serve to produce magnetism in a sensible degree ; and after an equally long 

 study of the actions of the transitory currents upon magnetized iron, it seemed to 



