GREEN P1C4MENT0F INTESTINAL WALLOP CHiETOPTERUS. 461 



percentages of the total amount of light passing through the slit 

 when not obscured, and having an aperture of '2 millimetre. 



The percentage is then written off on the chart by a dot 

 corresponding to the wave-length (right or left of vertical lines 

 of the chart), the percentage itself being given by the higher 

 or lower position of the dot in relation to the horizontal lines. 



The following is the report kindly furnished to me by Prof. 

 Engelmann, together with the table of measurements and the 

 charts given in Plates 36 and 37. It will be seen that by the 

 present method it is easy to calculate the form of absorption 

 curve for a greater thickness of solution from the observation 

 of that of a less, and vice versa, and that this has enabled 

 Prof. Engelmann to apply a satisfactory test to the accuracy of 

 his observations, which it must be remembered depend upon a 

 very delicate comparative judgment of the light intensity of 

 the two adjacent strips of spectrum, one of which is gradually 

 darkened by the turning of the micrometer screw until it is 

 judged to be exactly equal in light intensity to the other. 



Uteecht; 25th April, 1897. 

 . At last I am able to send you the results of the 

 quantitative colour-analysis of your Cheetopterin and Bonellin. 

 You will find them in the accompanying tables, and in graphic 

 form on the four charts of curves. 



The measurements were carried out with my micro-spectro- 

 photometer (' Zeitschr. f. wissen. Mikroskopie,' Sept., 1888'; 

 'Archives de Microsc.,' xxiii, 1888, p. 82, pi. iv; 'Onder- 

 zochingen gedann in het physiol. laborat. des Utrechtsche 

 Hoogeschool.,' 3, xi, 1889, pp. 39 — 49) with the use of an 

 Auer^s incandescent lamp as the source of light. The slit width 

 of the spectrum apparatus was in all cases 0*2 mm. The light 

 intensity thus given without absorption is taken as = 100 for 

 all wave lengths. Measurements were made of the intensity 

 of the light for a large number of X, after passage through a 

 plane-parallel layer of coloured solution of 5 and also of 2 mm. 

 in thickness. 



The measure of this intensity was in every case the width of 



