436 



Dr. G. F. Barker on a new 



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 sus- 

 pension of the needle, and moving with it. The scale and index 

 were placed in front of the condensing lenses of an ordinary 

 lantern; and their images were projected on the screen in the 

 usual way by use of the objective. This instrument is essen- 

 tially 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 skilful an experimenter as 

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



It was a tacit conviction that none of the forms of apparatus 

 now described 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 constructed by 

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

 an attachment to the ordinary lantern, is shown in the annexed 

 cut (fig. 1) . Parallel rays of light, from the p . , 



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, &c. may be placed, and many 

 beautiful experiments shown. To adapt this 

 vertical lantern to the purposes of a galvano- 

 meter, a graduated circle, photographed on 

 glass, is placed upon the horizontal conden- 

 sing lens. Above this, a magnetic needle, of 

 the shape of a very acute rhomb^ is suspended 

 by a filament of silk, which passes up through 

 a loop formed in a wire, stretched close 



* Journ. Frank. lust. S. 3. vol. lxi. p. 300, May 1871 ; Am. J. Sci. S. 3. 

 vol. ii. pp. 71, 153, July & August 1871 ; Quart. J. Sci. Oct. 1871*. In 

 Duboscq's vertical attachment, which was advertised in 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 

 vti) bright field. 



