348 Dr. L. Bleekrode. On the Determination of the 



The calculated probable error of the result amounts to 0-003. 



I will now adduce a complete series of observations taken from 

 preliminary experiments, which I made to become familiar with the 

 microscopical method. They serve also as an elucidation of the 

 manner in which the results recorded into the next chapter were 

 obtained. The following example bears on the determination of the 

 index of refraction of sulphuric ether, and was effected with the 

 cell mentioned above, and with a magnifying power of 14". 



Determination of the Height D. 



Focussing on the upper 



glass disk 29-00 24'34 20 "25 17 '63 14 '99 11 '58 



Focussing on the inferior 



glass disk 23-60 18 "92 14 '81 12'19 9'56 6'12 



5-40 5-42 5-44 5*44 5 "43 5 "41 



Mean D 5 '42 millims. 



Determination of the Correction for the Glass Effect. 



Focussing through the cell 3 '19 4 '67 6 '03 8 '79 8 '67 10 '05 



Focussing without the cell 2 '39 3 '86 5 "24 8 '01 7 '86 9-26 



0-80 0-81 079 078 0'81 079 



Mean '80 millim. 



Determination of the Index n. 



Focussing through the 



glass cell and liquid .. 2235 19 '00 1573 9 '88 14 "36 16 '83 



Correction for glass '80 0'80 '80 0-80 '80 0'80 



21-55 18-20 14-93 9 -08 13 '56 16 '03 



Focussing directly 20-13 16 '81 13 '53 7 '64 12 '10 14 '59 



1-42 1-39 1-40 1-44 1-46 1-44 

 Mean d=I'43. 



n _ D _ 5-42 

 D-d 5-42-1-43 



Temperature = 13. 



The light employed for the determination was that from a Bunsen's 

 gas-burner, the flame containing some natrium chloride ; the value, 

 therefore, refers to the D line. If it is compared to the result 

 obtained with other methods, for instance, that given by Lorenz*, as 

 1-3599 at 8 and 1'3521 at 21, leading by interpolation to 1'3569 at 

 about 13, it is obvious that the concordance is very satisfactory, and 



* The values are taken from the recently published " Physikalische und Chemische 

 Tabelle " of Landolt and Bornstein. The temperatures given in this paper always 

 refer to the centesimal scale. 



