COBALT CHLOiilDK 189 



of, approximately, 6 and 9 mm. The continuous background ended at 0.238^, 

 on the edge of the negative corresponding to the greatest depth of solution 

 in the wedge. 



Eye observations with the spectroscope showed the facts recorded below. 

 The depth of the cell was the same as when photographic exposures were 

 being made. For the solution of concentration 0.217 the spectrum began at 

 about 0.775,".. No bands were visible in the red. The band in the blue-green 

 was very faint and diffuse. 



The solution of concentration 0.759 also showed no bands in the red. The 

 most intense absorption seemed to be at 0.525;'., but the blue- violet side 

 of the region was so much more diffuse* than the yellow side, that not much 

 importance should be attached to this number. However, the spectroscopic 

 observations confirm qualitatively the spectrographic results. 



No bands could be seen in the red for the solution of concentration 1.409, 

 and absorption was complete from about 0.55S, to about 0.450//. . 



The violet has greatly dimmed. 



Similarly, the solution of concentration 2. 060 exhibited no bands in the red. 

 The band in the midst of the visible region extended from 0.57 1/* to, roughly, 

 0.448/*. As previously, the end of the band nearer the yellow was much more 

 definite than the end in the indigo. The violet was rather faintly transmitted. 



The mother-solution showed a series of weak bands in the red. Only the 

 orange and certain parts of the red were transmitted, and the light was con- 

 siderably weakened even in these regions. It would seem at first sight that 

 the last statement is inconsistent with the wave-length 0.432^, which the 

 spectrogram gives as the extreme least refrangible boundary of the region 

 of transmission in the ultra-violet and violet, when the well-known fact that 

 the average normal eye is sensitive to waves at least as short as 0.400/if is 

 taken into account. The apparent discrepancy is easily accounted for by 

 the following considerations : The photographic film exerted an integrating 

 action over a period of ninety seconds, whereas the retina does not add up 

 successive stimuli of the same spot for nearly so long a time. Moreover, the 

 dispersion of the spectroscope in the violet was much greater, and the spec- 

 trum no brighter than that of the grating in the same region. The spectrum 

 began about 0.773/(. From this point the intensity of transmission rose 

 gradually to a maximum, and then decreased to a faint minimum at 0.714^. 

 This faint absorption band was followed by another one at 0.676/z. The 

 spot of maximum brightness between these bands was at about 0.669/<. 

 Between 0.67/^and 0.59/ithe incident light had suffered less general absorption 



* Due allowance has been made for the fact that with a simple prism the dispersion 

 increases as the wave-length decreases. 



f Under the prevailing conditions the observer could see the air-line at 3995.1 with cer- 

 tainty. 



