238 Absorption Spectra. 



wedge. For obvious reasons this is only useful for very 

 opaque fluids, where, in fact, the position of the wedge to be 

 examined is near the centre of the lens. Such a form of 

 cell I have termed a " lenticular cell." The appearance 

 presented on looking through the upper lens at small 

 quantities of blood and chlorophyll, respectively, arranged 

 for examination according to the above method, is repre- 

 sented by P'igures 2 and 3, Plate VII. 



A more generally useful form of absorption cell is the 

 " conical absorption cell," which possesses the advantage 

 of forming a fluid wedge, or prism, available from centre to 

 periphery, and thus adapted for use with fluids of consider- 

 able opacity, as well as those of a more transparent character. 

 The diagram, Fig. 2, Plate VIII., is a representation of this 

 form of cell. The cone A ' which is constructed with its 

 angle of 90°,' is composed of crown-glass of very low refractive 

 density, so as to approach as nearly as possible to that of 

 water, the common solvent of most absorptive media. By 

 this arrangement the cell is made compensating, the refrac- 

 tion of the cone correcting that of the medium. In such a 

 cell the thickness of the medium is equal to its distance 

 from the apex of the cone. Where it is desirable to pass 

 the light transmitted through the absorption media of the 

 cell in a horizontal direction, as in the case where used with 

 many forms of spectroscopes, this can be effected by means 

 of a rectangular prism on the upper surface of the cone, 

 another rectangular prism being connected below the glass 

 plate B, so as to reflect the light horizontally through the 

 slit of the instrument. 



