RECORD OF CHANGES IN TERRESTRIAL MAGNETISM. 189 



SECTION XL 



ON THE CONTINUOUS REGISTRATION OF SMALL 

 CHANGES IN TERRESTRIAL MAGNETISM. 



83. General principle of photographic self-registra- 

 tion now usually adopted. Distinction of the magnetic 

 elements which are to be registered, and appropriate 

 positions of the recording apparatus. 



The object to be attained is, to make an impression 

 depending on the position of some part of the apparatus, 

 without contact, or friction, or mechanical resistance of 

 any kind. Nothing is so suitable for this purpose as 

 photography. If from a minute source of light (as a 

 lamp shining through a very small aperture) light falls 

 upon a concave mirror, or upon a plane mirror assisted 

 by a convex lens, which is firmly attached to a moving 

 part of the apparatus ; then a spot of light (the optical 

 image of the small source of light) may be formed at a 

 proper distance, and the motions of the moving part of 

 the apparatus will produce corresponding motions of the 



