378 



Prof. A. Gamgee and Mr. A. Croft Hill. [Jan. 31, 



of the optical activity of coloured solutions surprising when we con- 

 sider the much greater difficulties which encounter the observer, in 

 comparison to those attending the examination of colourless solutions. 



The Method Employed in the Present Research. 



"When a powerful beam of white light is passed through a stratum 

 1 cm. thick of a solution of Oxy- or CO- haemoglobin containing 

 0'9 per cent., the only region of the spectrum which is unabsorbed 

 is that which extends from B to a little distance on the red side 

 of D. It was therefore clear that the only light which could be 

 employed in the work before us was monochromatic red light, and that 

 in the place of one of the polarimeters commonly employed in our 

 laboratories and whose adjustments only permit of their being employed 

 with light of a definite wave-length (the half-shadow polarimeters of 

 the Laurent type being adjusted for use with monochromatic sodium 

 light), an instrument should be employed, the arrangements of which 

 permit of observations with light of any desired wave-length. 



In our first observations, we attempted to employ the lithium flame 

 as our source of light, but we were unable to secure by this means 

 either a sufficiently powerful or steady illumination. We subse- 

 quently employed, however, as a source of practically monochromatic 

 red light, the light of an arc lamp which had traversed Landolt's filter 

 for red rays. 



This light filter consists of a double cell, each compartment of which 

 has a depth of 20 mm. One compartment is filled with a solution of 

 hexamethylpararosanilin, a substance sold commercially under the 

 name of "Crystal Yiolet 5 BO." 0*05 gramme of this compound is 

 dissolved in a small quantity of alcohol and the solution is then 

 diluted with water to the volume of one litre. When light is made to 

 traverse a stratum 20 mm. thick of this solution, its spectrum consists 

 of a narrow red band and a broad blue-violet part. If, however, the 

 second compartment of the double trough contains a solution made by 

 dissolving 10 grammes of potassium chromate in 100 c.c. of distilled 

 water, the blue-violet is entirely absorbed and the spectrum of the 

 light which has traversed the two compartments of the light-filter 

 consists of a narrow strip, extending from A 718 to A 639/a/x, where it 

 ends abruptly. The mean wave-length (" optischer Schwerpunkt ") 

 corresponds to 665/x/x, the wave length of C being 656*3/x/z.* 



By means of the above method we secured a beam of red light 

 having a mean wave-length approximately the same as that of C and 

 of sufficient intensity to allow us to make observations on solutions of 

 haemoglobin containing ± 1 gramme in 100 c.c. of distilled water, the 

 tubes employed in different sets of observations being 100 mm. and 

 200 mm. in length. 



* Landolt, op. ext., pp. 387—390. 



