106 Prof. Wood on the Dispersion^ Ahsorption, 



This was found to be the case. An acute prism of quartz 

 was ground and polished, and one face of it brought in con- 

 tact with a small drop o£ nitroso fused on a strip of blackened 

 brass. One end of the strip was heated in a flame, the 

 nitroso remaining fluid as a result of the heat conduction 

 along the strip. The object of using the blackened brass 

 was to get rid of reflexion from the back surface of the nitroso. 

 The light reflected from the front surface of the quartz was 

 thrown to one side, owing to the inclination, so that every- 

 thing was eliminated except the reflexion from the quartz- 

 nitroso surfice. The spectrum of this selectively reflected 

 light is shown in fig. 5 (Plate VIII.). The minimum at wave- 

 length 36 will be seen to have disappeared, and a new one 

 will be found at 29 just about where we should expect it. 

 This minimum would doubtless be more pronounced were it 

 not for the fact that the absorption coefficient has at this 

 point a not inconsiderable value, which will cause the bound- 

 ing surface in question to have a higher reflecting power than 

 if both media were perfectly transparent. 



Absorption of Solid, Liquid, and Gaseous Nitroso. 



No determinations of the dispersion of the solid nitroso 

 have been made, owing to the difficulty of getting suitable 

 prisms: when the fluid prisms cool ofl" the nitroso crystallizes 

 in aborescent forms, and no longer transmits regularly. If 

 the prism is held close to the eye, and a lamp-flame viewed 

 through it, enormously deviated spectra are seen, due to the 

 formation of crystals having a much larger angle than the 

 prism. The substance in the solid condition is double- 

 refracting, the two spectra which every prism furnishes being 

 extinguished in turn by a revolving Nicol prism. The ab- 

 sorption-band of the solid nitroso was studied by photography. 

 A drop of fluid nitroso was pressed in a clamp between tw^o 

 hot plates of quartz, which were allowed to cool under pres- 

 sure. In this way a very thin film of the solid substance 

 was obtained. On photographing a spectrum through this 

 screen, it was found that the principal absorption-band had 

 broken up into two placed symmetrically with respect to the 

 band shown by the fluid. One of these doubtless belongs to 

 the ordinary, the other to the extraordinary ray. The centres 

 of the bands are at wave-lengths 36 and 46, while the centre 

 of the single band possessed by the liquid is at X = 43. 



In fig. 6 (Plate VIII.) we have two photographs of this 

 double band. 



The absorption of the liquid nitroso does not differ very 

 materially from that of its solution in glycerine, which I 



