56 



Scientific Proceedings^ Royal Dublin Society. 



This constancy of temperature was probably also advantageous in 

 maintaining the resistance of the galvanometer itself constant. 



The galvanometer employed is one of the Ayrton-Mather pattern, manu- 

 factured by the Cambridge Scientific Instrument Company. Its resistance 

 is 20'7 ohms. The deflection of the spot of light for one mioro-volt, when 

 the screen is 1 metre distant from the mirror, is 10 mm., and for one micro- 

 ampere 206 mm. A translucent screen was used to receive the spot of light 

 from the galvanometer-mirror, which was illuminated with a Nernst-larap. 



Fig. 3. — Plan showing disposition of apparatus. 



r thermostat containing the galvanometer G\ L galvanometer lamp; S galvanometer 

 transparent scale ■ K petroleum reversing key ; F freezing-bath on a separate table 

 lower than that «liich supports the rest of the apparatus. 



Where one observer is using the apparatus, it will be found convenient to 

 have the galvanometer leads so long that the petroleum key may be placed 

 close to the screen, while the freezing-bath and thermo-couple, etc., may stand 

 at a level 50 cms. below the screen and somewhat nearer the observer. This 

 disposition brings the key, tlie supports of the thermo-couple, and the 

 stirrer of the freeziug-batli close to the observer, and he is in a convenient 

 position for reading the position of the image of the cross- wire. 



Tlie apparatus should be set up, and the thermostat and galvanometer 

 adjusted, on the day before an observation is made. Once set up, no 

 readjustment should be necessary. 



The freezing-bath is contained iu the large cylindrical glass vessel, shown 

 in fig. 1, H, with thick walls. To prepare the bath the vessel is about a 



