Comparative Study of Chlorophyll. 397 



A piece of paper ruled into square inches and tenths has a 

 scale of wave-lengths ruled off along one edge, and the edge 

 at right angles to this has a scale corresponding to the scale 

 of the instrument marked on it. The value of the Fraun- 

 hofer lines on the scale of the spectroscope is observed, and 

 also their value in wave-lengths ; they are then marked in 

 their proper places on the scale with two x x. A curve is 

 then drawn through these marks as uniformly as possible. 

 Wiien a band or bright line has to be mapped out, all that 

 is necessary is to take its reading on the scale ; then, 

 knowing between what lines it is placed, we find its position 

 on the curve opposite which its wave-length is printed on 

 the right-hand edge." ^ 



In all the absorption-spectra which I have drawn I have 

 endeavoured to be as accurate as possible in giving to the 

 different absorption-bands their true extent and their respec- 

 tive degree of intensity of absorption. 



Spiroyyra. — Various species were tried, but I give the pre- 

 ference, when studying living chlorophyll, to slender species, 

 as they seem to let the light come through more uniformly. 

 Spirogyra longata, for example, serves the purpose admirably. 

 The first spectrum I attempted to define was that of chloro- 

 phyll in its living condition, and I proceeded thus : By means 

 of a glass rod a quantity of threads running parallel with 

 one another was lifted out of a basin ; the material assumed 

 the shape of a cone, and was carefully introduced into a 

 test-tube, sufficiently wHde to allow of its being filled with 

 water after the glass rod supporting the threads had been 

 laid across the mouth of the test-tube. The advantage of 

 this procedure is that we get a larger number of threads in 

 the upper part of the test-tube as compared with the lower 

 end, and therefore we may study readily the modifications 

 which the spectrum undergoes according to the amount of 

 light absorbed by various thicknesses of the material we are 

 examining ; all we require to do is to arrange the test-tube 

 in such a way before the spectroscope that a green pencil of 



* The wave-lengths" of Fraunhofer lines are, according to Angstrom, in 

 Rcchcrchcs sur le Spectre Solaire, Spectre Normal du Solid, p. 25. Upsala, 

 1868. 



A = 7604. B = 6868. C = 6562. 



D = 5892, E = 5269. F = 4860. G = 4307. 



