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Mr Glazebrook, On a Spectrophotometer. [Jan. 29, 



(1) All the ovals of the same intensity touch two of four certain 

 straight lines in points given by 



r = c 



(2) All the ovals of the same intensity are equal in area. 

 There are moreover a series of points of maximum and minimum 

 intensity determined by the intersection of the circles 



'2» + r 



r = c 



and the lines 



26 = y + - Y . 



(2) On a Spectrophotometer. By R T. Glazebrook, M.A., 

 F.R.S. 



The instrument was designed to determine the amount of 

 absorption of coloured solutions for the various rays of the 

 spectrum or to estimate by comparison with a standard solution 

 the amount of colouring matter in any given absorbing solution. 

 It consists of a long flat rectangular box (Fig. 2); 



at one end of this there is a slit A, the width of which can be 

 adjusted. The white light from a source behind the slit passes 

 through a collimating lens L placed at the distance of its own focal 

 length from A and falls as a parallel pencil on the set of direct 

 vision prisms SS'. The emergent beam is brought to a focus by 

 the second lens M and a pure spectrum formed on the end of 

 the box. 



A sliding piece fitted to this end carries a narrow slit B, and 

 through it any desired part of the spectrum may be viewed, C is a 

 second slit illuminated also by white light, the rays from which 

 after passing through the lens N fall on a plane mirror K, and 



