1S0.5.] 



Proceedings of Learned Societies i 



S45 



From this arrangement refults alfo the 

 fingulaily ft'-iking effeft of contaft of a 

 Jittle flip ot white paper applied luccef- 

 iively on paper, glafs, and cloth, of a 

 given colour. When thetranfparent body is 

 red, the opake white appears bluifli green; 

 if the ground be orange, it is decidedly 

 blue ; on a yellow groiird, a kind of vio- 

 let ; on a crimfon ground, green, &c. ; 

 always correfponding exaftly to tlie com- 

 plimentaiy colour. 



The knowledge of contrail may be 

 ufctuliy applied to thofe arts which are 

 employed on the fubjeiSl of colours. Tne 

 painter is aware that it is not a matter of 

 indifference what colour is placed near an- 

 other ; but when he is acquainted with the 

 Jaw to which their action on eicli other is 

 lubjeiled, he will know better what to 

 avoid, and how to dipofe his tints, fo as 

 to heighten the brilli.incy of that which 

 lie willies to bring forward. Coi trading 

 them together in I'uccefiion likewife affords 

 valunble indications of their nauire and 

 compofition. This the author himfelf has 

 put in prai51ice with advantage in his ma- 

 iiufaiffory of colours and paBer-haiigings. 



Considerations on contrails led Mr. P. 

 to ti)e e.xaminaiion of a very fingular cafe, 

 viz. the white appearance whicii a colour- 

 ed b.idy ibmctinics exhibits when viewed 

 through a glafs of the fame hue, and his 

 conclui'ion is, that the perception of white- 

 nefs in thife cafes is o-ving folely to the 

 aftion of contrails, by which the impref- 

 (ion of the colour is deadened or annihi- 

 lated ; while that of a certain degree of 

 brightnefo flill fl!bflii.^, and is noticed 

 from the oppofition of a greater degree of 

 obfciirity. Hence he gives a new defini- 

 tion of whitenefs : — " Wiiije is with le- 

 fpeft to us thefenfation of light, when no 

 particular colour predominates in it, or is 

 perceived in it." 



In another part of his Memoir the 

 author confiders the colouring of different 

 opake and tranfparent bodies, and inquires 

 what are the luminous rays which a given 

 coloured body is really cap:ible of reflefl- 

 ing or tranfmitting. In thecouifeof his 

 expeiimenl3 he difcovered that feveral 

 cpake fublirir.:es, of various natures and 

 of all colours, owed their coloured ap- 

 pearance to the following laws : — i. Each 

 of (he bodies always abfoibed the rays that 

 were complementary to tiie predominant 

 colour, z. In feme the abibrption in- 

 cluded, btfidis ihe complimentary fpecics, 

 oiheVs cullaieral lo this fpecics, and more 

 or lefi numerous. 3 The deeper a co. 



Monthly Mac. No. 135. 



lour is, the fewer fpecies of rays it re- 

 flefts. 



Speaking of the appearance of colour- 

 ed clouds, paitituiarly thofe we fee about 

 the rifing and fetting of the Sun, he fays, 

 this phenomenon has hitherto remained 

 without explanation. It is not, he afTerts, 

 owing to the refra6fion of the folar rays, 

 but to the fucceffive abforption of them, 

 when they ftrike on the inferior parts of 

 the atmofphere, which are loaded with 

 vapour. The quantity of vapours, and 

 even their nature, not being the fame every 

 day, produce correfponding difterences in 

 their tffefts. 



Commonly the firft rays attacked by 

 thefe vapours are the blue adjacent to the 

 violet. Soon after they attack the conti- 

 guous rays, gaining with mote rapidity 

 the blue pt .perly fo called } then the 

 green, the yellow, and thus proceeding to 

 the led. Hence the yellowidi, orange, 

 and red colours, exhibited by the clouds. 

 This period of tints, the evening for ex- 

 ample, dilplays itfelf gradually as the Sua 

 Epproaches the horizon. The fame hues 

 tinge teireftrial objeils, the part of the aic 

 nearcft the fun, and this luminary itielf. — 

 Accordingly, when we can receive its rays 

 on a priim, we perceive that the rays aftu- 

 ally ablorbed correfpond to the general 

 tir.t of the moment. 



Contrails may likewife render tlie co- 

 lour of the clouds complicated ; as for in- 

 ftance, when a great portion of the flcy 

 difplays its blue tint. There are fome 

 clouds the colour of which aiifes folely 

 from this caufe 5 and fuch may be feen at 

 times in the middle of the day, vvhen we 

 have a lof cy mountain at our back, or are 

 in any other fituation wliere the eyeisde- 

 fcended from the too powerful aftion of 

 the folar light, either diieft or reflefted; 

 but in this cafe the clouds have only a 

 yellov/ifh tinge, precife'y the complemen- 

 tary colour of Iky-blue. 



Sometimes the Moon is of a limilar co- 

 lour, when it is very high, a little before 

 or after the Sun pafTes the hojizon. It 

 appears thus, or even completely white, 

 when clouds varioufly coloured by the va- 

 pours of fun rife or fun-fet in the air at 

 the fame time, 



Laitly, from the irregularity of the 

 earth's lurface, and of the ftate of ths 

 atmofphere, the phenomena are liable to 

 be concealed or fubjcfttd to various inter- 

 ruptions. In our climate the colouring 

 of the clouds feldom reaches its laft ftage. 

 On fome evenings, whtn the fty is very 

 X X clear 



