230 Sir John Conroy. On the Refractive Index of [June 20, 



The prism was made by St.einheil; the value of its refracting 

 angle, as determined by six independent measurements, was 

 60 1' 42" 0-8". 



The prism was surrounded by a water-jacket, through which a 

 stream of brine, cooled by a freezing mixture, could be passed. 



Openings in the water-jacket'allowed the light which had passed 

 through the collimater to reach the prism, and the refracted beam to 

 reach the telescope. The temperature was ascertained by means of 

 a thermometer with its bulb immersed in the water contained in the 

 prism. 



The prism was filled with distilled water which had been recently 

 boiled and allowed to cool under reduced pressure. 



The determinations were made exclusively with sodium light ; it 

 had been originally intended to make observations with lights of 

 different refrangibilities, but it was found that, owing to the 

 brilliancy and constancy of the sodium light, it was not only far 

 easier to make observations with it, but that these observations would 

 certainly be more accurate than those made with light of other re- 

 frangibilities. 



The prism, not being in actual contact \vith_the water-jacket, cooled 

 very slowly, about four or five hours were usually necessary to reduce 

 its temperature from about 9 to a little above the freezing point. 



Owing to the experiments being made at temperatures different 

 from that of the room, and to the temperature of the prism con- 

 tinually, though slowly, altering, it was found impossible to make 

 the determinations by reversing the prism, and then taking half 

 the angle between the two positions of the telescope as the angle of 

 minimum deviation. 



Several series of observations were therefore made with the prism 

 in both positions of minimum deviation, and the differences between 

 these readings and those made when the axes of the collimator and 

 telescope were in the same straight line, gave the deviations. 



Seven sets of observations were made, and the results are con- 

 tained in Table I ; which gives the deviations for both positions of 

 the pi'ism, and the corresponding refractive indices for the various 

 temperatures. 



The angles of deviation differ so little from each other, that any 

 error in the determination of the refracting angle of the prism would 

 not make any difference in the relative values of the indices ; it 

 would, of course, affect their absolute values. The probable error 



calculated by the ordinary formula I 0'674 A / ) from the 



V V n(nl)/ 



measurements made of the angle of the prism (see above) is +0'8". 

 A difference of 1'' in the value of the refracting angle corresponds 

 to two units in the sixth place in the refractive index ; the probable 



