1883.] Mr Glazebrook, On a Spectrophotometer. 305 



being there reflected traverse the prisms and form a second 

 spectrum directly below the first. By adjusting the positions of 

 the lenses and the mirror K the lines in the two spectra can be 

 made to coincide. The light from A passes over the top of the 

 mirror and the two spectra are seen one above the other. A 

 concave lens enables the observer to focus distinctly the line of 

 separation. In front of each of the slits respectively are three 

 Nicol's prisms F, G, II 



F is fixed with its principal plane vertical, parallel therefore to 

 the slits and edges of the prisms, G has its principal plane 

 horizontal, while H is capable of rotation round a horizontal axis 

 parallel to the length of the box, P is a pointer fixed to the prism 

 H and moving over a graduated circle QR which is divided into 

 360 parts. The zero of the graduations is at the top of the cii'cle 

 and when the pointer reads zero the principal plane of H is 

 vertical. 



The Nicols F and G polarize the light coming through the 

 slits, the first in the horizontal plane, the second in the vertical. 

 The emergent beam is analysed by the Nicol H. When the 

 pointer reads zero or 180° all the light in the upper spectrum from 

 the slit A can pass through H, none of that from C is transmitted. 

 As the Nicol is rotated through 90° the quantity of light from A 

 which is transmitted decreases, while the amount coming from G 

 increases, and when the Nicol has been turned through 90° all the 

 light from G is transmitted and none from A. 



For some position then between and 90° the brightness in 

 the small portions of the two .spectra viewed will be the same. 

 Let the reading of the pointer when this is the case be 6. Let the 

 amplitude of the disturbance from A be a, that of the disturbance 

 from G be c, then clearly 



a cos 6 — c sin 0, 



and if I a , I c are the respective intensities 



I a* 



±? = - s = tan a 0. 



Now place anywhere between L and K a small rectangular cell 

 containing an absorbing solution. The upper spectrum will become 

 darker and the Nicol require to be moved to establish equality 

 again in the brightness. Let 6' be the new reading and I' a the 

 intensity of the light which now reaches the eye from A. Then 



^ = tan 2 <9'. 



Thus $.££. 



/. tan' 6 



