382 Mr. ft. W. Wood on the 



point is reached the surface-colour of the crystals will change 

 from brilliant green to a purple plum -colour, which will be 

 the surface-colour of the cyanin after it has solidified again. 

 The crystals fuse at about 135°; and as soon as it is evident 

 that solid particles no longer remain, an edge of the other 

 plate must be dipped into the fluid and the plate carefully 

 lowered until the opposite edge is in contact with the paper 

 strip. Both plates should now be lifted together, and the 

 cyanin edge compressed in a vice. This is the critical point, 

 and the right amount of pressure can only be learned by ex- 

 perience. If the pressure is too slight the fluid film will not 

 be squeezed out thin enough ; if too great, the plates of glass 

 will bend, and a prism of variable angle will be produced. It 

 is best to experiment with plates of different sizes and with 

 paper strips of different thicknesses. After the plates have 

 cooled they can be removed from the vice and examined. If 

 the prism is desired merely for purposes of illustration, the 

 plates had best be left in contact, as they protect the prism 

 from injury ; but if measurements are desired and the angle 

 of the prism required, the plates may be separated by a blow 

 from a hammer struck on the edge of one of them. The 

 cyanin prism will usually adhere to one or the other of the 

 plates, though sometimes half a perfect prism will stick to 

 each one ; on rare occasions it will split into . two layers, in 

 which case it must of course be rejected. 



I find that prisms of from 10' to 15' give the best results ; 

 the optical quality of the surface can be determined by re- 

 flecting the light from the collimator of a spectrometer, partly 

 from the cyanin and partly from the glass surface, the back of 

 the plate being rubbed over with a little grease to avoid con- 

 fusion arising from the third image. There should appear in 

 the telescope two images of the slit, one white due to the 

 reflexion from the glass, the other yellow due to reflexion 

 from the cyanin. If the prism surface is curved, the yellow 

 image will be broad ; but if it is found that by screening off 

 all but a strip of one or two millims. in width along the re- 

 fracting edge a fairly sharp image is produced, the prism 

 need not be rejected. By measuring the angular distance 

 between these two images we can compute the prism-angle. 



I have not succeeded in producing by this method a prism 

 sufficiently thin to transmit any appreciable amount of light 

 in the region of the absorption-band. But for observations 

 outside of this region I believe that far more accurate results 

 can be obtained than by the method employed by Wernicke 

 and Pfluger. A suitable prism having been formed, it should 

 be mounted on a black card provided with a narrow rectangular 



