202 Mr Purvis, On a method of estimating the amounts 



On a method of estimating the amounts of the oxides of 



Didymium and Erbium by means of the absorption bands of 



their solutions, and, its application to other solutions. By J. E. 

 Purvis, M.A.,-St John's College. 



[Bead 18 May 1903.] 



The following paper describes a method for determining the 

 amount of the oxides of Didymium and Erbium by means of the 

 absorption bands of their solutions, without separating them by 

 chemical methods from other elements with which they may be 

 united. 



Apparatus. 



The apparatus consists of a spectroscope A, which may be 

 a direct-vision one, or one arranged with a camera in order to 

 take photographs of the spectral bands. In front of the slit is 

 a glass tube B, of 6 inches length and of 1 inch internal diameter. 



^ 



The diameter must be equal along the whole length of the cylinder, 

 and the ends closed by flat glass firmly attached by screw joints 



