( '^'7 ) 



the isothermal lines etc., takes the Ibnn of llg. 2. In that case it 

 follows from A : 



hK«-i)- 



(D) 



Fis. 2. 



By expressing tg s as a fniiction of fg - from (C) one obtains (lie 

 relation (led need by Prof. Lokentz ; 



tg f = (^-1 - K) 



(^1 + K) 



Moreover from the geometrical solution here given the fact is again 

 brought to light that in general (he angle t|' is not equal to 90°; in 

 other words in this simple but exjierimeuhil way is proved by 

 occular demonstration the truth of the statement already made by 

 VoKJT, that the isothermal lines in crystals do not generally stand- 

 perpendicular to the direction of the flow of heat. 



Along the thermic axes however this is the case, because the 

 tangent lines at the ellipses are there directed perpendicularly^ to 

 these axes. 



From fig. 1 also follows the form of the break as a result of 



I hope soon to communicate the results obtained in the measurement 

 of crystals by means of tiiis method, together with a few observations 

 on the differences of these results with those, derived in the same 

 minerals by the usual methods of de Sénarmont and Röntgen. 



56 



Proceedings Royal Acad. Amsterdam. Vol. Vill. 



