OF ARTS AND SCIENCES. 



357 



For illiiminntion, the e(l<^e of a flame of a kerosene lamp was placed 

 in the focus of a system of condensers 4^ inches in diameter, and a 

 beam of ra3'-s was thrown from the distance of three feet upon the con- 

 cave mirror, which reflected them centrally upon the glass plate con- 

 taining the band. It was found that this illumination answered the 

 purpose ; for though the lines did not show the detail visible with mono- 

 chromatic light and sub-stage condenser, yet, being comparatively widely 

 separated, they were well adapted to measurement, when lines ruled 

 closely would have been measured with dilficulty. 



The following table contains the results in millimeters of the investi- 

 gation relative to periodicity of the values of one revolution. TI/q being 

 the mean of the first ten readings of the screw, and ^ being the sum 

 of the residuals of m^to24 from the mean value of one revolution. 



The following table contains the residuals of the separate readings 

 from the mean value of each revolution, also expressed in milli- 

 meters : — 



