proceedings: philosophical society 49 



fluorescent screen, against which it was difficult to focus the microscope. 

 The focusing is assisted by the use of monochromatic blue light. 



Messrs. Rosa and Vinal mentioned the use of the microscope in 

 connection with the determination of the ampere with the silver voltam- 

 eter, particularly with reference to the method of growth of silver 

 crystals and the effect on their appearance when impurities are present. 



Mr. Burgess described a micropyrometer which is convenient for 

 measuring melting points of microscopic samples. 



Historical aspects and the present status of the microscope were 

 discussed by Messrs. Wead, Fenner, Merwin, Nutting, Humphreys 

 and TiLLYER. The last emphasizing the use of the blue line of the 

 mercurj^ lamp as illuminator and suggesting an improved form of 

 projection eye-piece. 



The 716th meeting was held on December 7, 1912. Two papers were 

 read. 



A consistent theory of the origin of the earth's magnetic field: L. A. 

 Bauer, of the Carnegie Institution of Washington. See this Journal 3: 

 1. 1913. 



The earth inductor as an inclinometer: N. E. Dorsey, of the Carnegie 

 Institution of Washington. The speaker gave an outline of the theory 

 of the earth inductor with a continuously rotating coil and commutator. 

 When thermal electromotive forces exist in the circuit the setting of 

 the coil for a zero glavanometer deflection depends upon the speed; if 

 however, the axis of rotation lies in the magnetic meridian this effect 

 of variations in the speed is usually very small. A method for adjust- 

 ing the brushes so that commutation shall take place when the plane 

 of the coil is parallel to the inclination axis was described. When the 

 brushes are thus set, and the inclination of the coil is so adjusted that 

 the deflection of the galvanometer is independent, in both magnitude 

 and direction, of the direction of rotation of the coil, provided the speed 

 is the same in both cases, then the direction of the axis of the coil is 

 exactly the direction that would be taken by a perfect dip needle placed 

 with its pivots parallel to the inclination axis of the inductor. This is 

 true for all azimuths, and is independent of the presence of thermal 

 electromotive forces, provided that they are independent of the direc- 

 tion of rotation of the coil. For use at sea it is the mean value of the 

 galvanometer deflection that must be independent of the direction of 

 rotation. 



The 42d annual (717 regular) meeting was held on December 21, 

 1912, Vice-President Burgess in the chair. The meeting was devoted 

 to hearing the annual reports of the officers and to the election of officers 

 for 1913, the following being chosen: President, C. G. Abbot; Vice- 

 presidents, L. A. Fischer, W. S. Eichelberger, G. K. Burgess, 

 William Bowie; Treasurer, R. B. Sosman; Secretaries, W. J. Hum- 

 phreys and J. A. Fleming. General Committee: N. E. Dorsey, L. J. 

 Briggs, E. Buckingham, B. R. Green, E. G. Fischer, R. A. Harris, 

 F. A. Wolff, D. L. Hazard, R. L. Paris. 



R. L. Paris, Secretary. 



