566 Wisconsin Academy of Sciences^ Arts^ and Letters. 
was an instrument of the Konig-Martins^ type. It involves 
the use of polarized light and the equalization in intensity of 
the two rays by means of a rotating Nieol prism. The two 
beams which are to be compared are focused on lateral halves, 
Si and So (Fig. 1) of a divided slit. After passing through 
Fig. 1. Arrangement of optical parts of spectrophotometer. 
the collimation lens 0, and also the crown glass prism, C, for 
deviating reflected light, they are dispersed by the prism P. 
The light is then polarized by the Wollaston double image 
prism W and displaced by the biprism B so that of the eight 
spectra produced but two are used. These two are polarized 
in perpendicular planes. The eye-piece contains a Glam- 
Thompson prism, G, which acts as an analyser resolving all 
plane polarized light reaching it into two components vibrating 
perpendicular to each other and transmitting only that compon¬ 
ent vibrating parallel to the section. The circular field of view, 
L, is seen in the eye-piece divided into two halves. One half 
is illuminated with light from slit vibrating in a 
vertical plane and the other half with light from slit Sg 
vibrating horizontally. By rotating the Nieol the brightness 
of the two fields, which are illuminated by a narrow portion 
of the visible spectrum, can be equalized and from the angle of 
rotation the relative intensities of the original rays can be cal¬ 
culated. If the amplitudes of the linear light vibrations in 
the fields. A, and B, (Fig. 1) are represented by OA and OB, 
(Fig. 2) and ON is the plane of transmission of the analyser, 
then OA' and OB' will represent the amplitude of the light 
transmitted through A and B. So for equal intensity of A 
and B, 
OA cos 0 = OB sin 0 
OA 
or, — = tan 0 
^ Ann. der Phys., VoL 12, p. 984, 1903.: 
