26 Dr. A. M. Mayer on a new Lantern-Galvanometer. 



by rotating around the line of ^' the dip/^ as an axis, wire coils 

 at various distances and positions, and leading the induced mag- 

 neto-electric currents through the galvanometer. 



The lantern-galvanometer, which I will now proceed to de- 

 scribe, I devised on the 13th of last November; and as subse- 

 quent work with it has convinced me of its value in the lecture- 

 room, I have decided to give it this formal publication. 



Referring to the figure, M is a plane mirror inclined 45° to 

 the vertical. In front of this are the back condensing-lenses of 

 an oxyhydrogen lantern ; while the front lens of the condenser 

 is placed in a horizontal position at c, above the mirror. The 

 back condensing-lenses are of such curvatures that when the 

 calcium light is placed about two inches from the one nearest it, 

 a nearly parallel beam issues from them to fall upon M, thence 

 to be reflected to the upper condensing-lens at c*, on which 

 rests a disk of glass on whose border is photographed a divided 

 circle. In the centre of this disk is a short needle-point on 

 which freely rotates a magnetic needle, Above the needle is the 

 projecting lens L, the pencils from which are reflected in any 

 desired du-ection by means of the plane mirror R, which revolves 

 on a horizontal axis, and has also a motion in azimuth round 

 the axis of the lens-j*. 



The horizontal condensing-lens is 5 inches in diameter ; and 

 the magnetic needle is 4 inches long. With this arrangement I 

 have obtained sharp and bright images of the graduated circle 

 16 feet in diameter. 



To deflect this needle by means of an electric current, I place 

 as close to the condensing-lens as possible the two vertical wire 

 spirals S, S, formed of yig-inch copper wire of square section so 

 as to bring the convolutions as close together as possible. The 

 turns of the spirals are separated with very thin vulcanite ribbon 



* This arrangement of lenses, which is due to President Morton, gives 

 a bright and uniformly illuminated field free from coloration. 



In the Quarterly Journal of Science, October 1871, is a report of 

 Professor Morton's account of this invention ["the vertical lantern"], de- 

 livered before the American Institute, as follows : — " The original idea and 

 general plan of the instrument shown was, as the speaker stated, due to 

 Professor J. P. Cooke, of Cambridge — his own w^ork in connexion with it 

 being confined to the devising of a convenient mechanical arrangement of 

 parts, the improvement of the combination of condensing-lenses with the 

 reflecting lenses [mirror?] so as to secure a white and evenly illuminated 

 field on the screen, and the discovery that an ordinary silvered mirror 

 would serve for the final reflection as efiiciently as a metal speculum or 

 glass silvered by Foucault's plan, which are so difficult to obtain and keep 

 in order." 



t In a college course of lectures it is sometimes convenient to reflect the 

 image of circle and needle down on a white-covered table below the class. 

 The galvanometer can then be placed on the lecture-table. 



