Index of Refraction of Liquefied Gases. 345 



began with determining its height, putting it before the objective lens 

 of the microscope, and focussing first the objects on one of the glass 

 plates, then those on the other ; taking now the difference between 

 the two readings the value of D was obtained. I remark that in this 

 way the effect of the front glass plate (next to the objective lens) and 

 also of absolute errors in the division of the scale and micrometer 

 screw were eliminated. Putting successively the tube on other parts 

 of the scale, I could take a range of readings numbering six or eight, 

 from which the mean value was deduced. 



The determination of d, the apparent displacement, was somewhat 

 more complicated, a part of it being caused by the refraction of the 

 glass walls of the cell, and this was to be first ascertained. I found it 

 most convenient for this purpose to have the objects to be focussed 

 not on the glass plates of the cell, but on a separate glass slide placed 

 immediately behind it ; otherwise in taking different readings, when 

 working with a liquefied gas, I should have had the cell to be alter- 

 nately filled and emptied again. Yet the deduced mathematical 

 expression may also be used in this modified case. Considering in 

 fig. 3 an object / placed beyond the cell to be focussed through it, 

 either empty or filled, then, by the effect of refraction, it will be seen 

 apparently in e ; and when the angle fba is very small,* abf will be 

 almost a straight line, and again the equation is obtained 



mf_db_ D 

 me dc D d 



Therefore, when a complete observation of n for any liquefied gas 

 was to be made, a glass slide with test-objects was placed in front of 

 the microscope, the glass cell still empty (whose height had been 

 previously ascertained) followed close to it, and, putting the tube of 

 the microscope in a horizontal position, I focussed the objects first 

 through the cell, then, moving this aside, I focussed again ; a set of 

 such observations gave the mean effect of both glass plates of the cell 

 together. Now the liquefied gas was prepared in the glass tube con- 

 nected with the cell, and I took great care to have it completely filled 

 without any remaining gas-bubbles. I obtained this easily enough, 

 when maintaining the axis of the cell in a horizontal direction. 



To get the index of refraction of the liquid, I focussed through it 

 the test-objects ; I determined the position on the scale of the micro- 

 scope, corrected it for the glass effect, as described ; moving the cell 

 aside, I focussed again the test-objects directly ; the difference be- 

 tween the former and the latter reading gave the value of d, and the 

 height or D being already known, n is calculated. This method is 

 dependent on the condition that the cell is provided with parallel 



* With the greatest value of the angle of refraction, 5^, and with an index of 

 refraction ranging between 1'2 and 1'S, the obtuse angle amounts to 178 0- 9. 



