86 



Experiments on the Value of the Ohm. [June 15 T 



M. Cornu has shown that the quadruple group of rajs in the 

 magnesium spectrum may "become quintuple or sextuple, according to 

 the increased intensity of the spark employed. This is precisely what 

 might happen if one reversal by over-exposure were followed by a 

 second. Such reversals might be looked for if under the conditions of 

 the stronger spark the exposure of the plate were not shortened, 

 because the first and third of the four lines are stronger than the other 

 two, and they would therefore be the first and second to suffer reversal. 

 The reversal would split the lines in two, and hence produce the 

 appearance of a sextuple group. In order to ascertain whether this 

 might readily occur in the magnesium spectrum, some observations 

 were made with plates containing several photographs obtained by 

 different periods of exposure. Thus the first spectrum was the result 

 of ten seconds, the second of half a minute, and others various times 

 extending to half an hour. The quadruple group was not affected in 

 the way observed by M. Cornu, from which fact it would appear that 

 the division of the lines was caused by a reversal which was the 

 result of absorption of the central portion of the ray or rays. In 

 the two photographs obtained by the longest exposures, especially in 

 the last, the triplet b' between K and L became a quadruple group 

 by reason of the most refrangible line being split into two by a 

 reversal, the cause of which was nothing more than over-exposure. In 

 the quadruple group previously mentioned the lines were totally 

 reversed or not at all. This subject of reversal by over-exposure is 

 one well deserving the attention of those who are engaged in the study 

 of solar physics. Comparative exposures should be methodically 

 employed to confirm the accuracy of observations made entirely by the 

 aid of photographic representations of spectra. Especially is this 

 desirable when gelatine or other dry plates containing organic matter 

 are in use. 



III. " Experiments on the Value of the Ohm." Part I. By R. 

 T. Glazebrook, M.A., Fellow and Assistant Lecturer of 

 Trinity College, Demonstrator at the Cavendish Laboratory, 

 Cambridge, and J. M. Dodds, B.A., Fellow of St. Peter's 

 College. Part II. By K. T. Glazebrook, and E. B. 

 Sargant, M.A., Trinity College. Commuuicatecl by Lord 

 Rayleigh, F.R.S. Received May 24, 1882. 



(Abstract.) 



The method of the experiments is a modification of those of Kirch- - 

 hoff and Rowland. 



Two coils of copper wire of about 25 centims. radius, each containing 



