SCREW 511 



tral colors show through it, dark lines will appear, which are wavy if 

 there is a periodic error and straight if there is none. By measuring 

 the comparative amplitude of the waves and the distance apart of the 

 two lines, the amount of the periodic error can be determined. The 

 phase of the periodic error is best found by a series of trials after set- 

 ting the corrector at the proper amplitude as determined above. 



A machine properly made as above and kept at a constant tempera- 

 ture should be able to make a scale of 6 inches in length, with errors at 

 no point exceeding nnnnnrth of an inch. When, however, a grating of 

 that length is attempted at the rate of 14,000 lines to the inch, four days 

 and nights are required, and the result is seldom perfect, possibly on 

 account of the wear of the machine or changes of temperature. Grat- 

 ings, however, less than 3 inches long are easy to make. 



