Measurement of the Obliquity Factor of Diffraction. 621 



From an inspection of the original negative (or even of the 

 enlargement, cine regard heing paid to the reduction of 

 contrasts), it could be seen that the calculated values were 

 of the right order. 



The photographs were measured under a travelling micro- 

 scope. The instrument was one which had been constructed 

 by Messrs. Hilger for spectrum-photograph measurements. 

 The following tables show how nearly the observed positions 

 of the dark bands agree with the calculated values. 



Table I. 



Positions on the plate of 



The Direct image 



Observed. 



Calculated. 



75833 



7*5333 



1st Broad Dark Band 



72550 



7-2551 



1st Narrow ,, ,, 



71671 



7-1667 



2nd ,, „ „ 



71335 



7-1335 



3rd „ „ „ 



7-1035 



7-1041 



Table II. 



Positions on the plate of 



The Direct image 



Observed. 



Calculated. 



8-3626 



8-3626 



1st Narrow Dark Band ... 



8-6579 



8-6579 



2nd „ „ „ ... 



8-601)7 



8-6997 



3rd 



8-7339 



8-7347 



4th „ ,, 



8-7656 



87653 



