TRANSACTIONS OF SECTION A. 445 



that this new line in the nebula, as well as the two less refrangible lines which 

 had been observed by the eye, are produced by this substance. 



It is of much interest to compare the extreme differences of character of these 

 lines of hydrogen which are common to the white stars and to the nebula. lu 

 the stars these lines are very broad and winged at the edges, while in tjie nebula 

 they are of exceeding thinness and wholly free from a winged condition, indicating 

 great differences in tlie temperature and density of the gas as it exists in these two 

 orders of celestial bodies. 



I cannot say positively whether the lines of hydrogen between Ily and the 

 line fat 3730, which are present in the stars, are altogether absent in this nebula. 

 If they exist in the spectrum of the nebula they must be exceedingly faint rela- 

 tively to those present. I suspect a want of uniformity of the film at this part,, 

 and also beyond X 3730, and this may indicate possibly the presence of very faint 

 lines. Uud"er the most favourable co'nditious of illumination, I am almost certain 

 of faint lines at the positions of /* and H. In the diagram only those lines about 

 which there can be no imcertainty are inserted, and the new line is made broad 

 for the purpose of indicating its great intensity relatively to the other lines. 



In my laboratory experiments, I have succeeded in obtaining spectra of hydro- 

 gen in which some of the lines in this region are either absent or very greatly 

 reduced in intensity, but I have not yet obtained a spectrum which represents a 

 state of things precisely similar to that which obtains in this nebula. 



FRIDA Y, A VG UST 25. 



The following Papers were read : — 



1. On the Absolute Measurement of Electric Currents. 

 Bij Professor Lokd Rayleigh, F.B.S. 



The accurate absolute measurement of currents seems to be more difficult than 

 that of resistance. The methods hitherto employed require either accurate mea- 

 surements of the earth's horizontal intensity, or accurate measiu-ements of coils of 

 small radius and of many turns. If in the latter measurement we could trust to 

 the inextensibility of the wire, as some experimenters have thought themselves 

 able to do, the mean radius could be accurately deduced from the total length of 

 wire and the number of turns; but actual trial has convinced me that fine wire 

 stretches very appreciably imder the tension necessary for winding a coil satis- 

 factorily. Kohlrausch's method, in which the same current is passed through an 

 absolute galvanometer, and through a coil suspended bifilarly in the plane of 

 the meridian, is free from the above difficulty ; but it is not easy so to arrange the 

 proportions that the suspended coil shall be sufficiently sensitive, and the galvano- 

 meter sufficiently insensitive. In this method, as in that of the dynamometer, 

 the calculation of the forces requires a knowledge of the moment of inertia of the 

 suspended parts. 



When the electromagnetic action is a simple attraction or repulsion, it can be 

 determined directly by balancing it against known weights. In Mascarfs recent 

 determination a long solenoid is suspended vertically in the balance, and is acted 

 upon by a flat coaxial coil of much larger radius, whose plane includes the lower 

 extremity of the solenoid. This arrangement, though simple to think about, does 

 not appear to be the one best adapted to secure precise results. It is evident that 

 a large part of the solenoid is really ineffective, those turns which he nearly in 

 the plane of the flat coil being but little attracted, as well as those which lie to- 

 wards the further extremity. The result calculated from the total length of wire 

 (even if this could be trusted), the length of the solenoid, and the number of turns, 

 has an appearance of accuracy which is illusory, unless it can be assumed that the 

 distribution of the wire over the length is strictly uniform. It would appear that 

 all the turns of the suspended coil should operate as much as possible, that is, that 



