on the Optical Phenomena of the Atmosphere. 93 



To attain a more varied change of tint, Parrot* made use of 

 a rotating disc on which were laid sectors of prussian blue ; he 

 thus obtained a mixed colour capable of far greater modification. 

 His instrument was also divided into degrees. It seems, how- 

 ever, very difficult to obtain instruments of both descriptions 

 which are quite capable of being compared with each other. 

 After some experiments, we found it advantageous to apply the 

 colours in a different manner; and instead of expressing the 

 tint in degrees, to express it according to the proportions of the 

 mixture. We constructed two cyanometers, the first was of the 

 same form as that used by Parrot. A disc 20 centimetres in 

 diameter was covered with a layer of white lead, a substance 

 which, "When properly manufactured, possesses everywhere the 

 same degree of whiteness, whereas different descriptions of white 

 bleached paper vary greatly from each other in this respect. 

 The rim was divided into 100 degrees (1° being = 36 of the 

 usual divisions), and by means of these the whole surface was 

 divided into distinct sectors. This disc was fixed upon another 

 of pasteboard by means of little supports, which sustained the 

 centre and the rim merely. The rest of the space between both 

 discs was hollow. From three points situated 33-3 of the rim 

 divisions apart (120° in the common sense), a knife was drawn 

 along the corresponding radii. Through the slits thus formed, 

 blue and black segments could be pushed in until the required 

 portion of them was visible upon the surface ; the remaining 

 portion slid into the hollow space between the discs. The blue 

 segments were coloured by a layer of cobalt (oil colour) carefully 

 laid on; on the others was placed a layer of raven-black (oil colour). 

 These colours can be found everywhere, and exhibit such slight 

 deviations of shade tbat they may be regarded as constant. PL I. 

 fig. 3 exhibits the mechanical arrangement of this apparatus ; 

 tbe section of it is given at B. a is the plate of paper on which 

 the layer of white lead is laid; b is a disc of pasteboard parallel 

 to the latter ; at c are the sections of the supports which connect 

 both discs at the centre and rim; d is the projecting periphery 

 which carries the graduation ; e is a small cylinder of wood, 2 

 centimetres long, which is fast glued behind. Around this 

 passes a strap, which being pulled downwards, imparts a rotary 

 motion to the disc sufficiently quick, and of sufficiently long- 

 continuance, to permit of comparing the disc with the portion of 

 the firmament to be investigated. The screw /holds the instru- 

 ment fast to the upright which supports it during the rotation ; 

 at (j are plates used to strengthen the apparatus. 



In fig. A the surface; of the cyanometcr, as fitted for experi- 

 ment, is represented. The blue sectors partially cover the white 

 * Plujsik dtr Erdv, § 2/*, p. 102. 



