the Colours of Mixed Plates. 695 



Direction o£ observation perpendicular to 

 the plane of incidence. 



(ft -l)t. Angle of observation. 



obs. 8° 30' 23° 10' 



cale. 8 30 .21 10 



Angles of blurring. 



Blurring angle. 



6\ 



5X 



f obs. 1 

 \ calc. 1 



Fig. 2. Fig. 3. Fig. 4. 



30' 1° 40' 



1 18 1 20 



fobs. ... ... 3° 0' 



1 cale. ... ... 2 20 



Summary and Conclusion. 



In the present paper, the consequences of the theory of 

 the colours of mixed plates put forward by us have been 

 fully worked out, and quantitative data in its support are 

 furnished. It is shown that the results of theory and expe- 

 riment are in complete accord. The following are the 

 principal points brought out and explained in the paper : — 

 (1) The character of the diffraction-halo observed round 

 a light-source view T ed through a mixed plate of uniform 

 thickness ; (2) the changes in the appearance of a non- 

 uniform film with the angle of observation and especially 

 the blurring of the rings when viewed in certain directions, 

 and the observed influence of the plane of polarization of 

 the light on the position of the rings ; (3) the remark- 

 able asymmetrical character of the haloes observed with 

 obliquely-held plates ; and (4) the corresponding phenomena 

 with non-uniform films. 



In the final and concluding instalment of: the paper, 

 the special effects exhibited by partially or completely dry 

 films of albumen and some further studies of the phenomena 

 of mixed plates will be discussed. 



Calcutta, 25th March, 1921. 



