MA GNETIC REGISTRA TION. 179 



the Magnetic Conference, which was held at a meeting of the British 

 Association at Cambridge in 1845, there was a general expression of 

 opinion that it was a great drawback that no means had been hitherto 

 found for making the magnetic instruments record their own changes 

 of position. Attempts had been made to effect this by means of an 

 attached needle-point which was periodically impressed upon a surface 

 so as to mark it ; so that by that alteration in the position of the mark of 

 the needle, an indication of magnetic change would be obtained ; but it 

 was found totally impracticable. In point of fact, the amount of actual 

 force that is exerted is so very minute that it was quite clear that it 

 could actuate no pencil but one which moves without friction or 

 any mechanical resistance viz., a pencil of light. The desirability of 

 accomplishing this object attracted the attention of many persons to 

 it amongst others that of the late Sir Francis Ronalds and myself, 

 and the instruments which were devised for this purpose by both of 

 us are to be seen downstairs. In the apparatus of Mr. Ronalds the 

 plan pursued was to attach a screen with a slit in it to the magnet. A 

 light was burnt behind the screen, and as the magnet moved, of course 

 the screen would move by minute quantities, and the light transmitted 

 through the slit was allowed to fall upon a sensitive photographic 

 surface. But for various reasons which I need not go into now, this 

 system was found impracticable. The idea that suggested itself to my 

 mind was that of attaching a concave elliptical mirror, as you see here, 

 to the magnet. The same system is applied to all three instruments 

 such as I have described. A concave elliptic speculum is attached 

 which has its conjugate foci at about two and seven feet from the 

 surface of the mirror. A light, either a lamp or jet of gas, is placed at 

 a distance of two feet from this mirror. The light passing through the 

 small slit in the opaque chimney of the lamp or gas burner, as the case 

 may be, falls upon the mirror, and an image of that slit is formed at 

 a distance of about seven feet. The reason for using a slit, and not a 

 point, is that the image of the line of light is received upon cylindrical 

 lenses which contract an image which is about one inch and a half 

 long, into a narrow point not exceeding one-sixteenth of an inch in 

 width, and consequently the whole of the light is accumulated into a 

 very narrow space. That point of light falls upon a sheet of photo- 

 graphic paper which is placed round a cylinder, and the cylinder is 



