728 REPORT—1890. 
4, On Refraction and Dispersion in certain Metals. 
By H. E. J. G. pv Bors and H. Rusens.! 
Kundt’s method of experiment with very thin electrolytic metal biprisms was 
used in this investigation. In the first place measurements were made on red light 
obliquely transmitted ; from the deviations thus observed a process of integration, 
entirely independent of any particular optical theory, led to the empirical law 
connecting ¢ and ?z,,; these symbols denoting the inclinations of the wave-front in 
the air and the metal respectively to their common bounding-surface. Secondly, 
the dispersion was determined with all possible care, using four kinds of light 
defined by spectral lines. The following is a synopsis of the results :— 
I. Light, on passing from Fe, Co, and Ni (probably also from a number of other 
metals) into air, begins by following Snellius’ sine law for small angles of emission. 
II. The refractive index of such metals is mathematically defined as 
Hi (ain éjsin én). 
III. The actual metals deviate from ideal substances, supposed to possess the 
index thus defined in the following sense: to a given ti, corresponds a greater 
value of 2, or to a given 7a lesser value of 7 . 
The differences become more marked the greater the inclination, and are given 
empirically by the authors’ experiments ; for the three metals they decrease in the 
order Ni, Co, Fe. 
TV. The anomalous dispersion is illustrated by the following table of refractive 
indices :— 
Line | Li. a D F G 
7 
Tron ADEE, ri sont 3:12 2-72 2:43 2-05 
Cobalt 3-22 2-76 2°39 2°10 
Nickel 2-04 184 171 1°54 
5. On an Illustration of Contact Electricity presented by the Multicellular 
Electrometer. By Sir Witu1am Tomson, D.C.L., LL.D., F.R.S. 
In the multicellular electrometer, the force between the aluminium needles and 
the brass cells is modified by the ‘contact-electricity ’ difference between polished 
brass and polished aluminium. In the trial instruments made up to about a year 
ago, the result was scarcely perceptible; probably because care had not in them 
been bestowed to give high polish to the metallic surfaces. 
In the instrument as now made, differences of from two-tenths to three-tenths 
of a volt are found; averaging about + of a volt. 
The force by which + of a volt of difference of potential, on a difference of 
100 volts, bears to the force by which the same difference could be shown with the 
two metals in metallic connection, the ratio of (100+2)?—100? to (2)? or of 
800 tol. Thus the use of the multicellular electrometer gives a new and very 
interesting direct proof of Volta’s contact electricity. 
Some careful observations in Major Cardew’s new standardising laboratory of 
the Board of Trade, made by Mr. Rennie at frequent intervals during the last’ six 
weeks, have given doubled differences of from 65 to ‘6, seeming to show a slight 
tendency to decrease with time. 
6. On Defective Colour Vision. By Lord Rayunicu, Sec.R.8S. 
The existence of a defect is probably most easily detected in the first instance 
by Holmgren’s wool test ; but this method does not decide whether the vision is 
truly dichromic. For this purpose we may fall back upon Maxwell’s colour discs. 
1 Berlin Acad. Sitzungsber. July 24, 1890. 
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