778 Mr. F. C. Toy and Dr. J. C. Ghosh on the 



ratio of intensities incident, is determined for several times 

 of exposure. This variation of exposure will change the 

 values of y^ and y 2 , but the difference will remain constant. 

 The photograph shown in fig. 4 (PL XVII.) is taken over 

 the same range as that shown in fig. 3 and with the same 

 ratio of incident intensities, but with a different time of 

 exposure. Table A is typical of the values obtained : — 



Hatio of Intensities 23 : 1. 



Time of exposure 

 in minutes. 

 5 

 15 

 1-5 

 1-5 

 3 

 5 



Table A. 





3 : 1. Wave-length 308'8 ftp. 



t/l-t/2' 



Deviation of single 



in cms. 



observation from mean 



0-418 



0-008 



0-410 



0000 



0-395 



0-015 



0-424 



014 



0415 



0005 



0-398 



0-012 



Mean 



0-410 



0-009 



Average deviation of Mean = — ^ =0*004. 



Therefore 



(y l —y 2 ) = 0-410 + 0'004 cm. 

 Table B. 



Wave-length 

 in nfi. 



Gradation (increase of density per cm.). 



Mercury Lamp. 

 1JI 2 = 8-10. IJ1 2 =2W. 



M. F. Lamp. 

 I x /I 2 =3-10. 



660 



T-29 



1-42 



1-41 



1-41 



1-61 167 



1-61 



1-71 

 1-75 1-78 

 1-80 1-82 

 2-70 2-63 

 302 307 

 3-22 3-29 

 3-33 351 

 4-06 3-86 



1-23 

 1-21 

 1-25 



1-28 

 1-29 



610 



590 



579 



570 



560 



546 



492 



434 



408 



398 



391 



379 



368 



335 



312 



303 



297 



289 





