94 Dr. H. Schlagintweit's Observations in the Alps 



surface. As the radii of the sectors are the same as those of the 

 disc, the exposed surfaces of both arc proportional to the number 

 of degrees embraced by the circular contours. We have in the 

 present case — 



Blucj 15 parts 



Blue 2 13 ... 



Blue 3 5 

 The remainder of White 67 ... 



Sum 100 ... 



If the disc be now set in rotation, we shall obtain a mixed colour 

 the same as if we had blended — 



33 per cent, of blue, ** 



and 67 per cent, of white 



most intimately together. It will be afterwards seen, that in 

 this way a colour may be obtained, which, although it approaches 

 very near to that of the portion of the firmament under exami- 

 nation, still does not necessarily possess that tone which we 

 denominate the colour of the air. It would thus be possible to 

 attain the brightness corresponding to the position of the instru- 

 ment shown in the figure in another manner, that is, by omitting 

 blue 3 and setting in its place a sector of black (of course much 

 lighter). For the simplicity of the process and the comparability 

 of the results, we have found it more advantageous never to use 

 black as long as pure cobalt, which itself is a very dark colour, 

 was not lighter than the firmament*. 



As the setting up of the apparatus and the rotation of the 

 disc demanded considerable time, we found it convenient to have 

 an instrument similar to that of Saussure, that is to say, coloured 

 strips of paper, with which, however, neither the prussian blue 

 on the white paper, nor a division into degrees, was made use 

 of, but which was so arranged that the per-centage of cobalt could 

 be immediately ascertained. In the construction we proceeded 

 in the following manner : — 



A uniform cylindrical glass syringe was divided into 300 equal 

 volumes, and then filled alternately with Kremser white and care- 

 fully prepared pure cobalt (both oil colours and of the same con- 

 sistency) ; a scries of equal volumes of white and cobalt were 

 now placed beside each other on a palette. We had thus con- 

 stant colours, capable of being easily imitated by subsequent 

 experimenters. Oil colours, further, permit of being very inti- 



* Compare Arago's ingenious cyanometer, in which a plate of quartz, 

 cut perpendicular to the axis of the crystal, is used for the production of 

 the blue with which the colour of the sky is to be compared. — Annates de 

 Chintie, vol. iv. p. 98. 



