626 On Cyanine Prisms and Anomalous Dispersion. 



in the form of lumps of quite minute crystals. The old sample 

 had a different appearance, consisting of long needle-shaped 

 crystals not caked together. A certain amount of dexterity 

 is required to make good prisms, which can only be acquired 

 by practice. Small rectangular pieces of thin German plate 

 glass are prepared (measuring about 2 x 3 cm.), and a thin strip 

 cut from a visiting-card glued along the short side of one. A 

 piece of cyanine about the size of a coarse shot is placed near 

 the opposite side, and the edge of the plate heated over a small 

 flame until the dye fuses, holding another cover-strip in the 

 flame at the same time, in order to have both at about the 

 same temperature. The hot edge of the cover is now to be 

 brought down into the cyanine, and the plate gently lowered 

 until the edge rests on the strip of card. The plates must be 

 at once placed under pressure in a small clamp, where they 

 are to remain until cold. I find that the flat-jawed metal 

 clamp of one of Gaertner's laboratory supports gives the best 

 results. The pressure is to be applied close to the refracting 

 edge of the prism only, as shown in the figure. This is very 

 important. Experience is the only guide to the degree of 

 pressure required. With the new sample of cyanine the 

 removal of one of the glass plates, when this is desired, is much 



easier than with the old. For most purposes, however, I 

 prefer to leave the cover on ; cementing the two plates together 

 with sealing-wax. 



It will be found that there is a very narrow strip of clear 

 glass at the refracting edge, where the glass plates have come 

 into optical contact. This produces a diffraction-band super- 

 posed on the anomalous spectrum, but it is so faint that it is 

 not troublesome. 



It is usually necessary to turn the prism slightly to get 

 the green part of the spectrum ; that is, the incidence should 

 not be normal. (Examples exhibited.) The cyanine prism 

 should be held with the label-side towards the eye, and an 

 incandescent lamp or gas-flame turned edgewise viewed 

 through the slit. The refracting edge (which is to the left) 

 should be turned away from the eye a little, though the eye 

 must be brought close up to the aperture. The same thing 



