Index of Refraction of Liquefied Gases. 347 



ment of one. Its height should have been less than 0'5 centim., and a cell 

 which ought to resist strong pressure was not realisable in this case. 



I will now adduce the results of some determinations concerning the 

 thickness of a plane parallel glass plate, made with the microscope, 

 and also to verify the precision with a spherometer allowing to read 

 to a thousandth of a millimetre. 



Microscope. 

 (Magnifying power =30.) Spherometer. 



1 -11 millims. 1 "0745 millims. 



Mean. . 



Difference . . .0 '0054 millim. 



Calculating the probable error, its value is found for each single 

 determination as +O016, and for the result of a whole series as 

 0-006. 



Applying a much higher magnifying power, I could focus on the 

 very clearly defined strips on the scales of Lepidoptera wings ; the 

 results were still more satisfactory. As proof, I present the determi- 

 nation of the height of a glass cell, consisting of a short cylindrical 

 tube, having plane parallel plates on both ends. 



Microscope. 



(Magnifying power = 140.) Spherometer. 



5 '40 millims. 5 '413 millims. 



5 -42 5 -429 



5-44 5 '413 



5-44 5-427 



5 -43 5 -426 



5 -43 5 -432 



5-42 



5 -42 32 '540 



5-40 Mean 5 "423 



5-44 



54-24 

 Mean.. . 5 '424 . 5 '424 



Difference '001 



VOL. xxxvn. 2 A 



