Spectrophotometer of the Hiifner Type. 
283 
N and a glass rhomb AC — the Hiifner rhomb — is brought. 
The edge A is ground very sharp and bisects the slit gh. 
If we disregard the polarizing effect of the rhomb, the upper 
beam bounded by the rays sAg and thr, which illuminates 
the lower half of the slit, consists of natural light, and the 
beam bounded by the rays ugp, vAg, which illuminates the 
upper half of the slit, consists of light polarized in either a 
vertical or horizontal plane. Another nicol is inserted in 
the path of both beams. If we look into the eyepiece, we 
see two spectra, one above the other, and by rotating this 
nicol we can alter their relative intensity. 
The light which comes from the upper half of the slit 
must be plane-polarized before entering the second nicol, 
and the light which comes from the lower half of the slit 
must be unpolarized natural light. Bat it is partially 
polarized by both the Hiifner rhomb and dispersion-prism. 
Hiifner was perfectly aware of this, and in his improved 
instrument made by Albrecht he got over the difficulty by 
choosing the glass and angle of the dispersion-prism so that 
it removed the polarization produced by the rhomb (cf. article 
cited, p. 564). Messrs. Hilger have apparently rediscovered 
the same method of removing the polarization produced by 
the rhomb, and it is described at length by Mr. F. Twyman* 
under the impression that it is new. 
Any dispersion-prism will not do then. Its angle must be 
chosen so as to remove the polarization produced by the 
rhomb. 
I have recently designed a prism which performs the 
functions of both the rhomb AC and the Grlan-Thompson 
prism N, but which polarizes both beams in planes at right 
angles to one another, and thus can be used with any dis- 
persion-prism whatever (fig. 2). 
Fig. 2. 
ABC is made of glass ju D = 1*526, the sides AB, BC, CA 
* " Improvements in the Hiifner type of Spectrophotometer." 
F. Twyman, Phil. Mag. April 1907. 
