108 Dr Searle, Experiments with a plane diffraction grating 



been added to column 5, as suggested by Dr Wilclcen, and the results are 

 entered in column 7. 



The observed value of OK is a little low at 0° and 90° and a little high 

 at 45°. Probably the incident wave front was not accurately cylindrical. 



The last column gives ^^ ^^ calculated from tan ^^ — ^~~^ ^^^ ^i- The 

 difference between \//-2 and yjr^^ is too small to admit of measurement with simple 

 apparatus. 



