2908 G.F. Barker—New Vertical-lantern Galvanomeler. 
the instrument; and 2d, the angular deflection of the needle is 
r 
have made use, for the first time, of the excellent so-called 
vertical lantern in galvanometry. Upon the horizontal plane 
face of the condensing lens of this vertical lantern, Mayer places 
each side of the 
circle is drawn or photographed on the glass beneath the needle, 
and the image of this, together with that of the needle itself, 1 
projected on the screen, enlarged to any desirable extent. The 
defect of this apparatus, so excellent in many respects, seems to 
have been its want of delicacy ; for in the same paper the use 
of a flat narrow coil wound lengthwise about the needle, 1s 
recommended as better for thermal currents. Moreover, a year 
later, in 1873,+ Mayer described another galvanometer improve 
ment, entirely differént in its character. In this latter instru- 
ment, the ordinary astatic galvanometer of Melloni was made 
use of, an inverted scale being drawn on the inside of the shade, 
in front of which traversed an index in the form of a small 
acute rhomb, attached to a balanced arm transverse to the axis 
of suspension of the needle, and moving with it. The scale 
and index were placed in front of the condensing lenses of aD 
ordinary lantern, and their images were projected on the screen 
in the usual way by use of the objective. This instrument 1S 
* This Journal, II, iii, 414, June, 1872; Jour. Frank. Inst., III, Lxiii, 421, June, 
187 
| 4 This Journal, II, v, 270, April, 1873. 
