OF ARTS AND SCIENCES. 239 



Rutherford diffractiou plate, and a plate ruled by myself for the pur- 

 pose of investigating the errors of my screw. The measui'es were all 

 made with a -^^ objective, and an eye-piece micrometer, 200 to the inch, 

 the lines of which were about y^^o^ ^^ ^^ ^'^^^ ^^ breadth. Using a B. 

 oculai', the value of one division was found to be la^ocr °^ ^^^ inch. 

 With this arrangement, it was found easy to measure any given space 

 to 3^(y^x)¥ ^^ ^^ '"*^'^ with considerable certainty. To eliminate errors 

 in the micrometer, the same divisions were used in all comparisons. 

 In the Nobert plate, the width of the lines is about xb^^u^ ^^ ^^ i'^^'h ; 

 in the Rutherford plate, s^^jj^ of an inch ; and in my own, ^^^^o *^f 

 an inch. The space measured was ^^^ of an inch. 



Table I. contains the residuals obtained by subtracting the meas- 

 ure of each space from the mean of all the spaces. 



Table II. contains the residuals obtained by subtracting the error 

 of each space from the error of the next consecutive sj^ace. 



Table III. contains the periodic errors deduced from Table I. 



