Edinburgh New Philosophical Journal edinburghnewphil11855 Year: 1855 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- n
Edinburgh New Philosophical Journal edinburghnewphil11855 Year: 1855 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
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