Mat 21, 1909] 



SCIENCE 



831 



tated a correspondingly greater degree of regu- 

 larity — an accumulated error of one ruling in the 

 entire number being fatal. 



The ruling engine now set up in the Ryerson 

 Laboratory gives this necessary increase in per- 

 fection of ruling, so that the spectra are almost 

 free from ghosts and those of the higher orders 

 can be used. The resolving power is proportional 

 to the product of the total number of rulings into 

 the order. A photograph of a part of the Hg 

 spectrum was shown, in which the distance be- 

 tween two of the lines was only one two-thou- 

 sandths of the distance between the Na D lines, 

 and it was evident that lines separated by only 

 one half of this distance would be distinctly re- 

 solved. 



This ruling engine is the result of seven years' 

 work. The large ten-inch gratings are ruled on 

 metal to save the cutting edge of the diamond, 

 and weigh about thirty pounds. The greater part 

 of this weight is supported by a float in mercury, 

 only a small part pressing upon the ways. It is 

 moved along the ways by a screw with a large 

 head working by fine teeth in an automatically 

 actuated worm. This screw was made as perfect 

 as possible by long, careful grinding and the re- 

 maining errors which are of the order of the one 

 millionth part of an inch are automatically com- 

 pensated for by a slight tangential motion of the 

 worm. This motion is a function of the position 

 of the nut, determined empirically. 



On an Adjustment for a Plane Orating Similar to 

 Rowland's for the Conca/ve Grating: Professor 

 Gael Bakus, assisted by M. Barus, of Provi- 

 dence. 



By using two slides, one symmetrically normal 

 to the other, and observing on both sides of the 

 point of intersection, it is shown that many of the 

 errors are eliminated by the symmetrical adjust- 

 ments in question. The slide carrying the grating 

 may be provided with a focusing lens in front, 

 or again behind it, if the means are at hand 

 for actuating the slit, which is not sharply 

 in focus throughout the spectrum at a given 

 time, on the plane of the eye piece carried 

 by the second slide. It is thus best to use both 

 len'ses conjointly, the latter as a collimator and 

 the former as an objective of the telescope in 

 connection with the eye piece. The authors show 

 that a centimeter scale parallel to the eye piece 

 slide, with a vernier reading to millimeters, is 

 sufficient to measure the wave lengths of light to 

 few Angstrom units, while the wave lengths are 

 throughout strictly proportional to the displace- 



ments along the scale. The errors of the three 

 available methods and their counterparts are dis- 

 cussed in detail. 



The Electrometer Method of Standardizing the 

 Coronas of Cloudy Condensation: Professor 

 Gael Baeus, of Providence. 

 The author makes use of Thomson's method of 

 measuring the charge of an electron in terms of 

 the velocity of the ions and their number. The 

 latter, however, are determined from the angular 

 diameter of the coronas of cloudy condensation, 

 produced in a cylindrical fog chamber under given 

 conditions of rapid exhaustion. By providing the 

 chamber with a charged axial aluminum tube 

 closed within, and charged or not with sealed 

 tubelets of radium acting through the aluminum 

 walls in virtue of its beta and gamma rays, the 

 fog chamber becomes an electrical condenser with 

 a variable ionization. The electrical current 

 passing from core to the shell of the cylindrical 

 condenser may therefore be simultaneously meas- 

 ured by a sensitive electrometer. If it be assumed 

 that negative ions only are caught in the fog 

 chamber used and if the author's earlier and inde- 

 pendent results are employed for stating the nu- 

 cleation value of the coronas, the following data 

 are typical for the enormous ionizations produced: 

 Total number of ions per cub. cm. = 1,700,000, 

 385,000, 135,000. 



Corresponding value of ten thousand million 

 times the electrical charge ^4.4, 3.6, 3.9 electro- 

 static units. 



The author discusses the results critically and 

 shows that the displacement of ions during ex- 

 haustion is the most serious source of discrepancy. 

 Again that in a house in which a continuous light- 

 ing circuit terminates, it is absolutely essential 

 to determine both the positive and negative cur- 

 rent in the condenser separately, as they are liable 

 to be widely different. On the other hand, the 

 effect of gamma rays on the outside of the fog 

 chamber and of conduction currents is almost 

 negligible for a well-installed apparatus. 

 The Eleotrometric Measurement of the Voltair 

 Potential Difference between Two Conductors 

 of a Condenser containing a Highly Ionized 

 Medium: Professor Gael Baeus, of Providence. 

 Solar Activity and Terrestrial Magnetic Disturb- 

 ances: Dr. L. A. Bauee, of Washington. 

 This paper deals with the connections between 

 the various manifestations of solar activity, e. g., 

 sun-spots, and the so-called magnetic storms which 

 at times affect compass needles simultaneously all 

 over the earth by several degrees and even cause 



