142 Professor Forsers 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. Mexuonr 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 + six inches from Centre of the Pile. 
Dark Mean. Light. Ratio. 
od 5.2 86 : 100 
eg 
z : 
Deviations of Galva- 51 j,,5-9 6.0 83: 100 
nometer. ie \ 5.2 6.0 86 : 100 
ey: 6.5 83: 100 
5t 
* 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. he first 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. 
+ The oil lamp used when not expressly called “ argand,” was LocaTELtr’s 
Jamp with a solid square wick, which is what M. Mextonr employed. 
