TBANSACTIONS OF SECTION A. 703 



In order to commutate the coils R and X, the ebonite plate upon which the 

 connections C D E F are mounted is drawn up against the spring which presses the 

 contacts into their respective cups, until the guide pin is lifted out of its recess. The 

 plate can then be turned through 180° till the contacts are in the position shown 

 by the dotted lines marked CjDjEjFj ; the galvanometer is then brought to zero 

 by moving the contact key M into another position on its wire. 



The battery connections are shown at K K, and those of the galvanometer 

 at MO. 



In order to obtain a wide range of measurement, instead of using a lono- wire a 

 number of short bridge wires are provided, usually about ten, though if necessary 

 this number need not be considered the limit. 



The plate upon which each wire is mounted is detachable, as already stated and 

 in addition to the milled head N two steady pins are fixed in the base' of the com- 

 mutator so that it or any of the series can always be replaced accurately in posi- 

 tion with respect to the scale and key. 



A considerable variation of resistance of the short bridge wires shown will not 

 vitiate the accuracy of the measurement. 



This apparatus has been in use for the last five years substantially as now 

 shown. 



4. Note on a Galvanometer suited to Physiological Use. 

 By Dr. Oliver Lodge, F.B.S., and T. H. Nalder. 



Physiologists require galvanometers for exhibiting very small transient currents, 

 but they seem often to use highly damped galvanometers for the purpose. 



The first-named author, after some experiences with Professor Gotch, concluded 

 that a more suitable galvanometer could be designed, and accordingly sent a sketch 

 to Messrs. Nalder Brothers, who have carried it out. 



The main points are : — 



1. Extreme lightness and small moment of inertia of needle. 



2. Great intensity of magnetisation. 



3. Wire brought very close to the needle, so as to give a strong field without 

 excessive resistance, and to have many smaU coils in preference to few big ones. 



4. To avoid damping and to secure a long period by delicate suspension film 

 rather than by heavy needle. 



5. To use either a bee sting or some other sharp point in field of microscope for 

 reading, wherever a spot of light is inconvenient. 



The last condition has not been attended to yet, and perhaps biologists would 

 not care for it. The second-named author finds the sensitiveness of a galvano- 

 meter as above designed with 8 coils two or three times as sensitive as usual. 



5. 071 a Simple Interference Arrangement. 

 By Lord Rayleigh, Sec.R.S. 



If a point, or line, of light be regarded through a telescope, the aperture of 

 which is limited to two narrow parallel slits, interference bands are seen, of which 

 the theory is given in treatises on Optics. The width of the bands is inversely 

 proportional to the distance between the centres of the slits^ and the width of the 

 field, upon which the bands are seen, is inversely proportional to the width of the 

 individual slits. If the latter element be given, it wiU usually be advantageous 

 to approximate the slits until only a small number of bands are included. In this 

 way not only are the bands rendered larger, but illumination may be gained by the 

 then admissible widening of the original source. 



Supposing, then, the proportions of the double slit to be given, we may inquire 

 as to the effect of an alteration in scale. A diminution in ratio m will have the 

 effect of magnifying m times the field and the bands (fixed in number) visible 

 upon it. Since the total aperture is diminished wi times, it might appear that the 

 illumination would be diminished ni' times, but the admissible widening of the 



