2 LANKSHEAR, Quantitative Absorption Spectra. 



for convenience be called the standard beam. It passes 

 through the centre of the adjustable sector disc H and 

 falls on the prism F. It is reflected by F on to the prism 

 E from which it emerges horizontally, coming to a focus 

 on the spectrograph slit G. The prism B transmits an- 

 other beam, of equal intensity, upwards through the 

 the condenser C. Then the prism D reflects the beam 

 horizontally through the Baly tube or other containing 

 vessel for the solution under examination. The beam 

 comes to a focus on the slit immediately above and also 

 touching the standard beam. 



The Frame, The frame upon which the optical parts 

 are set consist of two aluminium uprights held together 

 by three stout brass rods. That nearest the source of 

 light carries a brass bracket on which are placed prisms B 

 and D along with the condenser C. This upright or end 

 piece carries the sector bush in which is mounted the 

 standard beam condenser. The other end piece carries 

 a brass bracket for the two prisms E and F. 



The Sector. The chief feature of the apparatus is the 

 sector. Instead of slots out near the periphery as in all 

 other sector photometers, there is a single semicircular 

 hole out at the centre with its diameter passing through 

 the centre of the wheel {Fig. 2). A disc having a corre- 



Fii 



sponding hole in , it can be rotated on the face of the 

 sector wheel, thus altering the aperture of the sector. 

 The latter is graduated logarithmically because the 



