106 ANNUAL OF SCIENTIFIC DISCOVERY. 



he had exhibited an apparatus, in which the electric light was used to 

 obtain upon a screen a magnified image comparable with that given by 

 the solar microscope itself. But in this instrument there was a great 

 inconvenience arising from the necessity of continually watching and 

 adjusting the charcoal points. This apparatus he lias since modified 

 so as not only to keep the poles at the same distance apart by a spon- 

 taneous action, but also to keep the radiant point immovable. These 

 results he obtains by the following arrangements. The two points are 

 pressed together by springs, but cannot move in that direction without 

 setting in motion a train of wheels, the last of which is controlled by 

 an escapement. The current of the apparatus passes around an 

 electro-magnet, the energy of which of course depends upon the in- 

 tensity of the current ; this electro-magnet acts upon a piece of soft 

 iron, which is pressed in the opposite direction by a spring. Upon 

 this soft iron is mounted the detent which checks the train of wheels 

 before mentioned, and the direction of the movement is such, that, 

 when the current becomes stronger, it presses upon the wheel-work, 

 and when it becomes weaker, releases it. And, -as the current be- 

 comes stronger or weaker according as the poles approximate to or 

 recede from each other, it will be seen that the poles become free to 

 approach each other as soon as their distance apart increases, but that 

 they can never come in contact, because the increasing strength of the 

 magnetism developed by their approach presents an insurmountable 

 obstacle, which removes itself as soon as the interpolar distance has 

 again increased. The approach of the charcoal points is therefore in- 

 termittent, but the periods of rest and movement succeed each other 

 so rapidly as to be equivalent to a continuous progression. 



M. Foucault requested the appointment of a committee to investi- 

 gate the originality of his invention, as it happens to be very similar 

 to that invented by Mr. Staite. The committee was appointed, and 

 reported " that the means invented by M. Foucault originated with 

 him, and were independent of those invented by Mr. Staite for the 

 same purpose." At a later meeting it appeared that M. Gaigneau, on 

 the 14th of January, 1848, had taken out a patent in London, in the 

 name of Mr. W. Petrie, for an apparatus which fulfilled the same 

 conditions in the same manner; but there was also in the same patent 

 a method of producing an intermittent light for light-houses, in which 

 the period of intermittence could be regulated beforehand. 



It would thus appear that, within a few months of each other, 

 Messrs. Foucault, Staite, and Petrie, each without the knowledge of 

 the others, contrived methods for producing a constant electric light, 

 which were almost exactly similar. 



EXPERIMENTS WITH THE LIGHT OF THE VOLTAIC ARC. 



AFTER giving the description of his apparatus mentioned above, 

 M. Foucault continued : " We thus obtain by means of my instru- 

 ment arcs of all kinds, which are persistent, and which can by the 

 aid of lenses be thrown upon a screen, so that their physical appear- 

 ance can be contemplated, or upon a linear diagram, so that they can 



