RICHARDS AND MARK. — THERMAL EXPANSION OF HYDROGEN. 129 



The evaluation of these divisions was made by repeatedly noting the 

 number of divisions passed through in changing the setting of the cross- 

 hair from coincidence with one mark of the meter-bar to coincidence 

 with another. It was found that the distance from the telescope to the 

 object observed must not vary more than half of one millimeter in order 

 to preserve this value to the required accuracy. The distance from the 

 upper iron point of the Lord Rayleigh barometer to the object lens of 

 the telescope — 775.0 mm. as given by concordant measurements — was 

 adopted as standard whenever the telescope was used. The following 

 table contains the comparisons of dial divisions with the length of 

 separate successive lengths of two millimeters each. The measurements 

 were made from two independent adjustments of the focus, with very 

 careful measurement of the distance between the meter-bar and the tele- 

 scope. Each of these figures represents the mean of these measurements. 



The slight differences between these measurements are due in part to 

 necessary experimental error, and in part to the slight inaccuracies of 

 the divisions measured ; but the probable error of the average is so small 

 vol. xli. — 9 



