110 Messrs. V. H. Veley and J. J. Manley. 



VI. Effect of Temperature on Refracting Angle. 



In the course of the work numerous determinations of the refracting 

 angle were made at different temperatures ; of these the following 

 pair may be cited, each value being the mean of several de- 

 terminations : 



Temperature. Eefracting angle. 



11-6 60 6' 4" 



19-0 60 6' 2" 



This difference is quite insignificant. It is possible that differences 

 in the refracting angle at different temperatures, noticed by previous 

 observers, may be due not to any alteration in the prism by itself, but to 

 the expansion of the wax or other cementing material, as to the thick- 

 ness of which and the method of spreading it details are not given. 



VII. -Temperature. 



In the more recent experimental work upon refractive indices the 

 prism has generally been enclosed in some form of jacketing arrangement 

 through which a stream of water at about the required temperature 

 was allowed to flow. Some of the arrangements suggested increase 

 the difficulty of manipulating the prism in any direction, while for 

 every case it appears probable that, if observations are required at a 

 temperature considerably above that of an ordinary temperature, an 

 optical strain might result from convection currents from the hot 

 jacket, and thus observations at a higher might not be comparable 

 directly with those at a lower temperature. 



Some observers have remarked upon the number of constants 

 required for an empirical equation 



to fit the observations ; possibly the explanation may be found in such 

 a strain as described above. 



As in this particular investigation the substance under examination 

 may begin to decompose at a temperature of about 30, the plan was 

 adopted of altering the whole temperature of the room so that not 

 only the prism, but also the lenses, telescope fittings, micrometer 

 scales, &c., should be affected pro rata as regards their individual co- 

 efficients of expansion.* 



It was judged that, when the thermometer in the prism contents 

 and a second of the same order of delicacy placed on the rotating 

 table registered the same temperature for a time longer than that 



* TJiis method of working was adopted by van der Willigen in determinations of 

 the variation in refractive index of glass prisms with temperature. 



