on the Optical Phanomena of the Atmosphere. 95 



raately mixed, and of being uniformly laid on the surface, which 

 in our case was that of weakly-sized Bristol-board. In this way 

 we obtained fifteen divisions of a scale ; the first of which was 

 white, and the last pure cobalt. The difference from one division 

 to the next was a matter of indifference in the application of the 

 instrument, as the per-centage content of cobalt and not the 

 number on the scale was noted. The increase of cobalt from 

 one division to that next to it was not uniform. We endeavoured 

 to have the differences of shade from leaf to leaf tolerably alike ; 

 and bere we remarked, tbat a uniform addition of cobalt becomes 

 less appreciable when a considerable quantity of the colour is 

 already present. In the last leaves, therefore, we used a greater 

 proportion of cobalt than in the first ; the immediate object of 

 the latter was to render the instrument more uniform. 



Cyanometrical experiments are, in general, determinations of 

 the brightness rather than of the colour ; it is, however, of some 

 interest to investigate the shades of the latter a little more closely. 

 A mixture of white and cobalt cannot fully accomplish this. The 

 most direct way of proving this, is by looking at a landscape 

 painted in oil, where only white and blue are used in the treat- 

 ment of the sky. An addition of red or yellow is always neces- 

 sary. As the shades of colour exhibit considerable changes, it 

 seemed to us not unimportant to determine their relations, at 

 least approximately, for different elevations. The colour which 

 is generally added to complete the sky tone is light ochre (hy- 

 drate of iron) ; this unfortunately is a colour which, strictly 

 speaking, cannot be regarded as constant in all manufactories. 

 But the smallness of the quantity used, which never exceeded 

 11 per cent., served to render the disturbance arising from this 

 less appreciable. 



In the construction of this second scale, and of a third for the 

 colour of the clouds, we have been assisted by the advice of that 

 distinguished landscape-painter, M. A. Zwengauer of Muuchen, 

 to whose kind and friendly support we take this opportunity of 

 expressing our deep obligation. 



The basis of the tricoloured cyanometer, consisting of a union 

 of cobalt, white and ochre, was formed by three different mix- 

 tures of the last two colours. The first consisted of 20 parts of 

 white and 1 part of ochre; the second of 20 parts of white and 

 2 of ochre; and the third of 20 parts of white and 3 of ochre. 

 To each of these separately were added 1, 8, 12, 20, and 50 parts 

 of cobalt, BO that for every tone we had live divisions of the 

 scale j we had, therefore; fifteen divisions in all. In the forma- 

 tion of a Bcale forjudging of the colour! of the clouds, such a 

 simple process could not be followed. The most suitable proce- 

 dure appeared to us to be that of imitating the most marked 



