Spectrophotometer of the Hiifner Type. 
285 
■usually fit a much smaller nicol, thus causing a large loss of 
light. In the eyepiece the rays come closer together, and a 
smaller nicol may be used. An ocular nicol with divided 
circle, reading telescope, and screens for cutting out the 
regions of the spectrum not under observation was therefore 
made by F. Schmidt and Haensch, Berlin, to replace the 
ordinary eyepiece. The prism for attaching to the slit was 
made by R. & J. Beck, Ltd. Its angles do not require to 
be made accurately, the only point necessary being that the 
axis of the Iceland spar should be parallel to the edge A. 
It is extremely important that the slit prism be set with the 
axis of the Iceland spar perpendicular to the refracting edge 
of the dispersion-prism. If that is not the case, the two 
beams are not polarized in and perpendicular to the plane of 
incidence when they enter the dispersion-prism. Their 
planes of polarization will then be rotated, and the positions 
in which the ocular nicol extinguishes the upper and lower 
spectra will not be at right angles to one another. The 
correct position of the slit prism may thus be found by 
trial. 
If it is preferred to keep the one beam unpolarized, as in 
the ordinary Hiifner spectrophotometer, then a prism of the 
form shown in fig. 3 might be used. ABC and BFD are 
glass, FDEC is Iceland spar. There is now no light lost by 
reflexion in the nicol. Owing to the space saved, wider cells 
might be used and the absorption of more dilute solutions 
measured. 
The limitation of the size of the aperture causes no difficulty 
in practice. 
With reference to the accuracy obtained, it is not possible 
with a Hiifner rhomb to get the two spectra under comparison 
to touch one another so sharply as in the case of a Lummer- 
Brodhun cube or a biprism such as is used in the 
Koenig-Martens spectrophotometer, as my experience of the 
Phil. Mag. S. 6. Vol. 15. No. 86. Feb. 1908. X 
