Barker.] 



442 



[May 7, 



images were projected on the screen in the usual way by use of the 

 objective. This instrument is essentially the same in principle as the 

 mirror galvanometer ; but it cannot be as sensitive as the latter, while it 

 is open to the same objection which we have brought against this— the 

 objection of unintelligibility. In the hands of so skillful an experimenter 

 as Mayer, it seems, however, to have worked admirably. 



It was a tacit conviction that none of the forms of apparatus now de- 

 scribed would satisfactorily answer all the requirements of the lecture 

 above referred to, that led to the devising of the galvanometer now to be 

 described, which was constructed in February of the present year. Like 

 the first galvanometer of Mayer, the vertical lantern, as improved by 

 Morton,* forms the basis of the apparatus. This vertical lantern, as con- 

 structed by George Wale & Co., at the Stevens Institute of Technology, 

 as an attachinent to the ordinary lantern, is shown in the annexed cut, 

 figure 1. Parallel rays of light, from the lantern in 

 front of which it is placed, are received upon the 

 mirror, which is inclined 45° to the horizon, and are 

 thrown directly upward, upon the horizontal plano- 

 convex lens just above. These rays, converged by 

 the lens, enter the object glass, and are thrown on 

 the screen by the smaller inclined mirror placed 

 above it. The upper face of the lens forms thus a 

 horizontal table, upon which water-tanks, etc., may 

 be placed, and many beautiful experiments shown. 

 To adapt this vertical lantern to the purposes of a 

 galvanometer, a graduated circle, photographed on 

 glass, is placed upon the horizontal condensing lens. 

 Above this, a magnetic needle, of the shape of a very 

 acute rhomb, is suspended by a filament of silk, which 

 jii_jgSil passes up through a loop formed in a wire, stretched 

 close beneath the object glass, and thence down to 

 Fig. 1. the side pillar which supports this objective, where 



it is fastened by a bit of wax, to facilitate adjustment. The needle itself 

 is fixed to an aluminum wire, which passes down through openings drilled 

 in the scale glass, the horizontal lens, and the inclined mirror, and which 

 carries a second needle near its lower end.f Surrounding this lower 



* Jour. Frank. Inst., Ill, Ixi, 300, May, 1871 ; Am. J. Sci., Ill, ii, 71, 153, July, Aug., 

 1871; (4.uai-. J. Sci., Oct., 1871. In Duboscq's vertical attachment, which was advertised inv 

 his catalogue in 1870, the arrangement is similar, except that the beam received upon 

 the mirror is a diverging one, and consequently the horizontal lens is of shorter focus. 

 A total reflection prism, placed above the object glass, throws the light to the screen. 

 The instrument gives a uniformly illuminated but not very bright field. 



t After the new galvanometer was completed and had been in use for several weeks. 

 I observed, in re-reading Mayer's first paper, a note stating that the idea had occurred 

 to him of u.=ing an astatic combination consisting of two needles, one above the lens and 

 the other below the inclined mirror — the two being connected by a stiff wire passing; 

 through holes in the condenser and the mirror. The plan of placing the coil round the 

 lower needle does not seem to have suggested itself to him. Indeed, it does not appear 

 that the arrangement he mentions was ever carried into practical effect. 



