398 



Prof. R. W. Wood on Uneven Distribution of 



experiments were confined to wire gratings, and dealt merely 

 with the amount of light directly transmitted under the 

 two conditions. 80 far as I know, polarization has never 

 been introduced into the theory of gratings. 



It occurred to me that polarization might prove to be the 

 key to the explanation of the very singular behaviour of the 

 grating of which I am writing. Experiment proved this to 

 be the case, for it was found that the singular anomalies ivere 

 exhibited only when the direction of vibration {electric vector) 

 was at right angles to the ruling. On turning the nicol 

 through a right angle all trace of the bright and dark bands 

 disappeared. The bands are naturally much more conspicuous 

 when polarized light is employed. 



We will now examine in some detail the appearance of 

 the spectrum at different angles of incidence. In fig. 1 we 



63 

 Angle of 

 Incidence. 



62 61 60 59 



4 12 

 2 37 



15 



5 







5 



1 15 



1 53 



2 38 



5 45 



Fig. 1. 



57 53 



51 50 

 _l 



I 



No. 1. 



No. 2. 



No. 3. 



No. 4. 



No. 5. 



No. 6. 



No. 7. 



No. 8 



No. 9 



No. 10 



have the appearance of the spectrum for ten different angles 

 of incidence. The position of the dark and light bands in 

 the spectrum was determined by employing sunlight, and 

 using the Fraunhofer lines as reference-marks. The wave- 

 lengths are indicated at the top of the figure, and the angles 

 of incidence at the left. Beginning with No. 1, we have the 





