222 H. -EMO GL OB IN. 
front of the slit, a metallic box enclosing the following optical parts. (In order 
to facilitate our description, these are shown in the following diagram, Fig. 32. 
Fig. 32. — Schematic representation of the path followed by the rays of 
light before entering the slit of the collimator of Hufner's spectro- 
photometer. — After Krtiss. 
which indicates also the path of the rays passing through the glass trough 
containing the coloured solution.) 
Placed centrally, in the position shown in the diagram, is an oblique parallel- 
epiped of flint glass, with two of its diagonally-opposed angles in a line with 
the optic axis of the collimator. This admirable optical contrivance (which 
is known in Germany after the optician who devised it as "der Albrecht'sche 
Glaswiirfel oder Glaskbrper") refracts light falling on its two anterior faces 
so as to alter its direction, as shown in the diagram, and as will be after- 
wards referred to. Placed anteriorly to the lower half of Albrecht's body 
is the small Nichol's prism <1. Corresponding to the upper part of the glass 
body is a composite glass plate e, with perfectly parallel sides. This plate is 
formed by cementing together two glass wedges, of which one is of clear glass 
and the other of smoke-tinted glass, and can be moved from side to side by 
means of a special arrangement. According to the position of this plate it 
will absorb more or less light. The purposes of these various parts are 
sufficiently obvious from the diagram; aa represents the absorption-trough for 
containing the coloured liquid to be spectrophotometrically investigated. In 
the lower half of the trough is seen the Schulz's cube (b) ; r and /•' represent 
two parallel beams of light falling on the anterior surface of the trough. The 
lower beam (/■) traverses in its path the Xiehol prism ('/), and is polarised; 
falling then on the adjacent surface of the parallelopiped, it is deviated so 
as to fall upon the upper half of the slit. The upper beam may or may not 
meet in its path the composite plate c previously referred to, and to which 
reference will again be made. This beam is so deviated as to fall upon the 
lower half of the slifc. After traversing the structures just described, two 
beams of light fall upon the slit — a polarised beam on the upper half and a 
non-polarised beam on the lower. 
2. The telescope. — A very ingenious arrangement, which is indicated by 
a separate drawing in the centre of Fig. 31, permits of the precise 
position of the telescope in reference to the prism being determined, and 
consequently of the most accurate determination of the position of any line 
in the spectrum. The reader is referred for details to Professor Hufner's 
original description. At the distal end of the telescope is the object glass, 
next to it is a Xichol's prism, the rotation of which is measured on a graduated 
circle by the help of a vernier. In the focal plane of the eyepiece is a 
modification of Yierordt's eyepiece slit, permitting of any determined spectral 
region being exactly isolated. For further details as to the construction and 
adjustment of the spectrophotometer, the reader is referred to the original 
