\vrrn THE MICROSCOPE. 225 



describe this subject in full detail, and therefore will refer to my paper 

 in the Proceedings of the Royal Society (1867, vol. XV, p. 433) for 

 a complete account of the general method, and of those laws which 

 indicate the presence of one or more colouring matters in a solution. 



321. Method of Measuring the Position of Absorption Bands. 

 In order to measure the exact position of absorption bands. &c., 

 seen in spectra, I have contrived a small apparatus, which gives an 

 interference spectrum, divided by black bands into 1 2 parts, all of 

 equal optical value. It is composed of two Nicol's prisms, with an 

 intervening plate of quartz, about '043 inch thick, cut parallel to the 

 principal axis of the crystal, the thickness being so adjusted, that the 

 sodium line is exactly at 3^, counting the bands from the red end 

 towards the blue. I have placed such standards in the hands of 

 Mr. Browning, and Messrs. Beck, who have undertaken to prepare 

 others like them. 



The characters of this scale will be better understood from 



fi g- 3 2 3, pl- L. 



In the spectrum microscope this spectrum is, as it were, in direct 

 contact with that under observation, and the position of any absorp- 

 tion can be easily measured to within T ^th part of the width of the 

 whole spectrum. Such a system of measurement enables us to adopt 

 a method by means of which spectra can be easily described in notes, 

 or printed by means of ordinary types. In order to express the 

 intensity of absorption, I make use of the following symbols : 



Not at all shaded Blank space. 



Very slightly shaded . . . Dots with wide spaces. 



Decidedly shaded . . . Dots closer together. 



More shaded .... Very close dots. 



Strongly shaded, hut so that "V 



a trace of colour is still I - - - Three hyphens close, 

 seen J 



Still darker Single dash. 



Nearly black Double dash. 



Except when specially requisite, only the symbols . . . 

 are employed for the sake of simplicity, and then as signs of the 

 relative, rather than of the absolute, amount of absorption ; and it 

 is assumed that there is a gradual shading off from one tint to the 

 other, unless the contrary is expressed. This is done by means of 

 a small vertical line over the figure (sec No. n, p. 227) which shows 

 that there is a well-marked division between them. Definite narro\v 

 absorption bands are indicated by * printed over their centre. This 



Q 



