TRANSACTIONS OF SECTION A. 583 



In the marine voltmeter, an oblate spheroid of soft iron is suspended in the 

 centre of, and with its equatorial plane inclined at about 40° to, the axis of a coil 

 of fine copper wire, by means of a stretched platinoid wire. When a current is 

 passed through the coil the oblate of soft iron tends to set its equatorial plane 

 parallel to the axis of the coil, and this tendency is resisted by the rigidity of the 

 suspension-wire. • • j- 



The lamp-counter is a tangent galvanometer with special provision tor prevent- 

 ing damage to its silk tibre suspension, and for allowing the constant to be readily 

 varied by the user to suit the lamps on his circuit. 



4. Supplement to a Report on Optical Theories. 

 Bij R. T. Glazebrook, M.A., F.B.S.See Reports, p. 208. 



5. Description of a Map of the Solar Spectrum. 

 By Professor H. A. Rowland. 



6. Exhibition of Negatives of Photographs of the Solar Spectrum. 

 By Geo. Higgs. 



The author exhibited some negatives obtained by himself with the aid of some 

 temporary instruments, the camera objective being a double convex spectacle-lens 

 of 6 ft. 4 in. focus, and the camera a plain wooden box of the same length. The 

 collimator is only 13 inches long, having a slit exactly 1 mm. in length and about 

 -Mb mm. in breadth ; the light passes through four prisms of light flint, each 

 havino- an angle of 45° ; the object-glasses are 1^ in. in diameter ; the whole being 

 of excellent quality, by Mr. Browning. The detail is such that eighty-one lines may 

 be counted between H' and H- : a strong line, which he had not previously ob- 

 served, is distinctly observable in the centre of H^ ; the nebulous iron line 4045 is 

 resolved into about five nebulous lines. The space between 4101— that is, A— and 

 the strong pair on the less refrangible side of it, which is vacant space in Ang- 

 strom's map, contains thirty-five lines, all distinctly visible. 



Ilford :^-plates have been used, and in a length of about 10 cm., from G nearly 

 to H., between 900 and 1,000 lines may be counted. A great number of bright 

 lines and spaces are observed, which are probably due simply to the absence ot 

 lines. 



7. On the Period of Rotation of the Sun as determined hy the Spectroscope. 

 By Henry Crew. 



These observations were made with the large spectroscope designed by Pro- 

 fessor Rowland for the Johns Hopkins University. With the aid of a good con- 

 densing-lens and mirror of plane parallel glass silvered on the front a sharp image 

 of 12-5 mm. diameter was obtained. First one limb and then the other was placed 

 on the slit by rotating the condensing lens about an axis parallel to the axis of the 

 collimator. The displacement of the lines in the spectrum was measured by a 

 micrometer screw in the eyepiece. The grating used was one of Professor Row- 

 land's, having 14,436 lines to the inch, and giving superb definition in the 4th 

 order. 



The angle between the solar axis and the slit of the spectroscope was obtained 

 by calculating the parallactic angle ; observing, by means of a rotating prism, the 

 angle between the projected image of a plumb-line and the slit ; and tlien adding 

 the sum of these two to the position-angle of the sun, all three being taken with 

 their proper signs. • j -? oo oo 



Twenty-four series of observations, ranging in solar latitude from 23 to 3o 

 degrees, give a final value of 2-827 ± '02 statute miles per second for the relative 

 ■velocity of the eastern and western limbs of the sun at the equator. The reduction 



