Simple Variation Compass. 79 



merits analogous to Kater's circle, or indeed any form of theo- 

 dolite, may wish to make observations of magnetic declination. 



It consists of a collar A, fitted so as to slide without much 

 friction upon the object-end of the telescope of the theodolite 

 with which it is to be used; an arm B, projecting in front of 

 the telescope, furnished with a fine steel point C ; and a small 

 collimating magnet LF, supported on an agate cap, which 

 turns on the point C. The best form for the collimating mag- 

 net, would, I conceive, be that of a hollow steel cylinder, car- 

 rying at one end a lens L, and at the other a cross of spider- 

 lines F, as represented in the figure, — a construction which 

 has been adopted in various magnetic declinometers. In the 

 instrument made for me by Mr Adie, instead of the cylinder 

 shown in the figure, there are two steel plates, each 5 inches 

 long, 0-3 broad, and 0-02 thick, placed parallel to each other, 

 and connected at the ends by light frames of brass ; an ar- 

 rangement which answers exceedingly well. One of these 

 frames carries the lens L, and immediately behind the other, 

 and between the plates, so as to be out of risk of injury, is 

 placed a diaphragm carrying the cross fibres F. The lens is 

 not achromatic, but as its aperture is only 0*2 inch, while its 

 focal length is 4*7 inches, the image of the cross fibres formed 

 by it is tolerably well defined. I should recommend, however, 

 the adoption of an achromatic lens of greater aperture, and 

 shorter focal length than that which I have described, and the 

 hollow cylindrical magnet instead of the parallel steel plates ; 

 for the cylindrical magnet will admit of the lens and cross 

 lines of the collimator being more firmly fixed in their places, 

 while at the same time they will be less liable to derangement 

 from handling the magnet. It is scarcely necessary to ex- 

 plain, that the rays of light proceeding from the cross fibres, 

 which are placed in the principal focus of the lens, are ren- 



