142 Professor Forbes on the Refraction 



the axes parallel, and then with the axes crossed, which diffe- 

 rence might yet be made up by the secondary radiation from the 

 heated tourmaline, which was constantly becoming more intense. 

 Such at least appeared to be the chief source of error, which I 

 am particular in stating, because I afterwards discovered that 

 M. Melloni had been led to the very same conclusion as I at 

 first was, and had published it. # 



20. When I proceeded to verify my results by a series of suc- 

 cessive observations, under the two conditions of axes parallel 

 and axes crossed, so as to eliminate any error from a constantly 

 progressive change, I perceived my mistake. As this illustrates 

 the method by which almost all my observations have been re- 

 duced, I shall give an example. Two measures of intensity in 

 the position where least light was transmitted, which is marked 

 Dark, have their mean taken, which is then compared with the 

 intervening observation in the position of greatest illumination, 

 which is marked Light. These tourmalines we may call A and B. 



1834, Dec. 4. — Oil Lamp-f six inches from Centre of the Pile. 





Dark 



Mean. 



Light. 



Ratio. 





' *4 



} 4°5 



5.2 



86 : 100 



Deviations of Galva- 

 nometer. 



5 



} 5.0 

 } 5.2 



6.0 



6.0 



83 : 100 

 86 : 100 



1 



1 Si 



} 5.4 



6.5 



83 : 100 



* In operating with tourmaline, and also with other substances which transmitted 

 directly but little heat, and which, therefore, required to be placed near the source 

 of heat, in order to get distinct results, I have always found that the small differ- 

 ences of effect of which I was in search, became gradually less as the process of 

 conduction advanced. The^r,^ result was generally the best marked. This effect 

 may be compared to the destruction of the phenomena of diffraction in light, by the 

 interference of other undulations than those producing the phenomenon sought. 

 Such interfering waves would, in this case, proceed from the secondary radiation 

 of the interposed bodies. But there seems also some inaptitude in the pile to ac- 

 commodate itself to reiterated and very slightly different alternations of temperature. 



f The oil lamp used when not expressly called " argand," was Locatelli's 

 lamp with a solid square wick, which is what M. Melloni employed. 



