198 PEACTICAL PHYSIOLOGY 



collected by a second tube, the telescope, and the eye-piece of this 

 should be arranged to receive parallel rays from the surface of the 

 prism. The eye-piece is frequently fitted with cross wires, if so, the 

 eye-piece should be first adjusted so that they are seen distinctly. 



It will be found that the telescope has some lateral movement 

 so that the vertical cross-wire can be made to coincide with any 

 required part of the spectrum. 



The third tube will contain a small scale which can be focussed on to 

 the surface of the prism and will then become reflected along the axis 

 of the telescope. It will be necessary first to adjust the movable 

 end of the scale-tube so that the scale is in focus, and it may be 

 necessary to move it laterally so that the scale is brought into the field 

 of the telescope. 



The scale must remain in a fixed position during any series of 

 observations. 



Construction of a chart for determining wave-lengths of bands on the spectrum. 



With the scale in fixed position, notice the position on the scale of the sodium 

 line, and the lines observable when the sodium flame is replaced by one coloured 

 with a salt of strontium, calcium, lithium, barium, caesium, and potassium. Take 

 observations of the positions of about twelve of these lines, the wave-lengths 

 of which are known. Obtain the values corresponding to these wave-lengths, and 

 on a piece of paper, ruled in squares, plot out their position, regarding the 

 abscissae as degrees of the scale and the ordinates as wave-lengths. Draw a curve 

 through the several points. The wave-length of any part of the spectrum can 

 now be ascertained. Observe where such a part intersects the scale, follow the 

 ordinate corresponding to the degree of the scale to the point of intersection with 

 the curve, and a line parallel to the abscissae line will indicate the wave-length. 



Having arranged the spectroscope so that the scale is illuminated and 

 visible through the eye-piece, and the slit is illuminated by a light 

 (an argand or incandescent burner) placed about one foot off; notice 

 the position of the D-line on the scale. If sodium chloride be sprinkled 

 into the illuminating flame, the D-line will be manifest, but a better 

 method is to arrange between the illuminating flame and the slit a 

 Bunsen flame in which asbestos soaked with a strong solution of 

 sodium chloride is placed. 



A piece of glass tubing about two feet long may be taken, which has the lower 

 six inches bent back to form an angle of 60 with the main stem. This is filled 

 with 6 p.c. solution of sodium chloride. The short arm is then plugged with 

 asbestos. At the end of the long arm is a short piece of rubber tubing 

 clamped fairly tightly. This tube is held by a burette clamp, and the projecting 

 asbestos can be allowed to just touch the Bunsen flame. In this manner a 

 constant D-line is furnished. 



1. The visible Spectrum of Oxyhaemoglobin. Take some defibrin- 

 ated blood which has been thoroughly shaken with air, and dilute 



