234 Dr. Alex. Hodgkinson on 



place, and not a great one there. The variation in strength 

 of the absorbing medium, therefore, exerts little, if any, 

 effect on the position of the lines in the primitive spectrum. 



To complete the enquiry, it remains to determine whether, 

 or not, a variation in the angle of the hollow prism exerts 

 any effect on the position of the primitive lines. For the 

 purpose of deciding this, I constructed a prism with a re- 

 fracting angle of 30°5'. The spectrum depicted in Plate 

 V. was obtained, and the primitive spectrum seen beneath 

 determined. The line A is at A -576, and B at A -543. Again, 

 the variation is only in the third decimal place, and not a 

 great one. It may be objected that a difference in the 

 angle of the hollow prism of 4 35' is not a great one, but 

 the spectrum represented in Plate III. is that of pure 

 blood, observed in a prism of not more than io°, and the 

 variation here is still only in the third decimal place. We are, 

 therefore, justified in saying that the variation in the angle 

 of the hollow prism exerts little or no effect on the position 

 of the absorption lines of the primitive spectrum. 



That a fair idea may be obtained of the uniformity of 

 these results, on Plate VI. the four primitive spectra 

 obtained as described are arranged in series under the same 

 scale. A fifth spectrum is added, which is a spectrum 

 obtained by taking the mean position of the lines in the 

 other four. This I regard as, within very narrow limits, the 

 true primitive spectrum of oxyhaemoglobin. 



It may here be pertinently enquired what the variation, 

 slight though it is, is due to. I believe such variation to be 

 due to two causes. Firstly, the having no special appliance in 

 the eye-piece of the spectroscope, to arrange along the edges 

 of the absorption bands so as to accurately indicate their 

 points of convergence. Secondly, some probable inequality 

 in my hollow prisms, which were home-made. The first 

 want necessitated the graphic mode of representation con- 

 stituting the diagrams, and there is sure to be some 



