134 Mr. Satyendra Ray on the 



recognized that its use in other cases may require modifica- 

 tion of the details of the instrument, experimental procedure, 

 and of the reduction of the data to the most useful form. 



One problem to which this method has already been applied 

 with considerable success is the measurement o£ the increase 

 or decrease in the visibility of objects resulting from the use 

 of colour filters. The change of visibility in such cases is 

 obtained by the adjustment of the selective absorption to fit 

 the requirements and the condition under which they are 

 used. The enhancement of visibility by use of colour filters 

 usually depends upon their ability to increase either the hue 

 or saturation contrast or both. The increase of visibility in 

 such cases can be determined quantitatively by first making 

 one reading in the usual way without a filter and one after 

 having inserted the filter between W and L l (fig. 7). 



The complete interpretation of such results requires an 

 extension of the theory to cover the evaluation of visibility 

 in terms of hue and saturation contrast and cannot be pre- 

 sented at this time. In conclusion, the author desires to 

 express his sincere thanks to Dr. 0. E. K. Mees and 

 to Mr. Lindon W. Bates for their many helpful sugges- 

 tions and constant encouragement given through the course 

 of this investigation. 



Research Laboratory, 



Eastman Kodak Company, 

 April 21, 1919. 



IX. A Note on the Equivalent Shell of a Circular Current. 

 By Satyendra Ray, M.Sc, B.A., Lecturer in Physics, 

 Canning College, and Senior Science Master, La Martiniere 

 College, Luchiow*. 



§ 1. Introduction. 



^|^HE magnetic induction of a circuit at any point is 

 _1_ identical in magnitude and direction with that due to 

 a magnetic shell bounded by the circuit, the strength of the 

 shell being numerically equal to the current. The equiva- 

 lence is, however, true only for points not close to the mag- 

 netic shell. [Vide Maxwell's ' Electricity and Magnetism/ 

 vol. ii. §§482-484.] 



The shell is defined as magnetic matter magnetized in a 

 direction everywhere normal to its surface. This together 

 with the law of refraction of lines of induction, viz. 



= ' t, and the law that for a tube of induction 



* Communicated by Prof. D. N. Mallik, Sc.D., F.R.S.E. 



