540 HENRY A. ROWLAND 



72 72 T2 TV 72 72 72 72 72 72 /2 72 72 72 72 



t/0 "1 "2 "8 "4 "5 "6 "7 "8 "9 "10 "11 "12 "13 "U 



This table shows how the primary line weakens and the ghosts 

 strengthen as the periodic error increases, becoming at 2nJV a 2-405. 



tt 



It then strengthens and weakens periodically, the greatest strength 

 being transferred to one of the ghosts of higher and higher order as 

 the error increases. 



Thus one may obtain an estimate of the error from the appearance 

 of the ghost. 



Some of these wonderful effects with 20 to 50 ghosts stronger than 

 the primary line I have actually observed in a grating ruled on one of 

 my machines before the bearing end of the screw had been smoothed. 

 The effect was very similar to these calculated results. 



DOUBLE PERIODIC ERROR 



Supposing as before that there is no overlapping of the lines, we 

 have the following: 



Places. Intensities. 



[/ (ba^ ,/ (toirif } Primary line. 



= ,,. - [7i (ba^J ,7 



(. Ghosts of 1st order. 



/> 



= fi ~ [J (ba^) /! (ba^J 



