222 



HEMOGLOBIN. 



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 Hiifner's spectro- 

 photometer. After Kriiss. 



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 Glaskorper") 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 d. 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 r 1 represent 

 two parallel beams of light falling on the anterior surface of the trough. The 

 lower beam (r) traverses in its path the Nichol prism (d), and is polarised ; 

 falling then on the adjacent surface of the parallelepiped, 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 slit. 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 Hiifner's 

 original description. At the distal end of the telescope is the object glass, 

 next to it is a Mchol's prism, the rotation of which is measured 011 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 



