148 Royal Society : — Mr. H. C. Sorby on Analysis of Animal 



objection, but is prepared with far greater difficulty. The sides should 

 be perfectly parallel, and the thickness about *043 inch, and gradu- 

 ally polished down with rouge until the sodium-line is seen in its 

 proper place. This must be done carefully, since a difference of 

 1 Q lo a inch in thickness would make it decidedly incorrect. I have 

 prepared such plates, corresponding to my own, and placed them in 

 the hands of Mr. Browning and of Messrs. Beck, so that any one 

 wishing to adopt a similar scale may be able to do so more accu- 

 rately. 



The two Nicol's prisms and the intervening plate are mounted in 

 a tube and attached to a piece of brass in such a manner that the 

 centre of the aperture exactly corresponds to the centre of any of the 

 cells used in the experiments, which are all made to correspond in 

 such a manner that any of them, or this apparatus, may be placed on 

 the stage and be in the proper place without further adjustment, 

 which, of course, saves much time and trouble. 



4. Symbols used to describe Spectra. 



In order to describe spectra in my note-book or in print, I have 

 devised a simple notation, employing types in constant use. Instead 

 of writing an account of this system, I here give a printed illustra- 

 tion, which will show that by this means it is easy to give in a single 

 line all the essential particulars which would otherwise require a long 

 and tedious description, or a number of drawings and woodcuts. 

 Without some such method of measuring and recording spectra it 

 would be almost impossible to carry on extensive inquiries. 



The intensity of the absorption is expressed by the following 

 types : — 



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 but so that a trace 1 _.. Three h ^ ^ 



of colour is still 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 (see p. 1 50, No. 11), which shows that there is a 

 well-marked division between them. Definite narrow absorption- bands 

 are indicated by * printed over their centre. This will be better 

 understood by a description of the spectrum of deoxidized hsematin. 



