344 



Proceedings of Learned Societ'teu 



[Nov. 1, 



Pruflian blue, and fal-ammoniac, are dan- 

 gerous to the neighbours only from want 

 cf due precautions, and that' the care of 

 government fhouid extend only to an. ac- 

 tive and enlightened fuperinterdence, hav- 

 ing for its o' je8s the improvement of their 

 proctfi'es, and of the management of the 

 fire, and the maintenance <if cleanlinels. 

 3dly. That it would be wor;hy a good 

 and wife government, to make leguiations 

 prohibiting the future ellablifliment of any 

 mnufaiSlurc, the vicinity of when is at- 

 tended v^ith any eflcniial inconvenience or 

 danger, in towns and near riwtilling- 

 houle», without fpecial authoiiiy pre- 

 vioufly obtaiied. In this clafs may be 

 compiized the mmulaflorits o'i poudrette 

 (d;y night-foil), leacher, and ftaich j 

 foumleries, meltiig-hourcs for tallow, 

 flaiighter-houles, rap-warfchoufes, m.inu- 

 failoiies of Prufnan-blue, varnifh, glue 

 and fal-ammoniac, pot eiies," &c. 



Thefe c nchifpons were adopted by the 

 Inftitutc, and addrelTed to Government, 

 with an invitat on to make thcin the bails 

 of its decifijris. 



M. C. A. PiUEUR has lately laid be- 

 fore the Inliitute a Memoir, initled 

 *' Confiderations o . Colours, and offeveral 

 oF thtir lingular Appiaancts." He fets 

 out from the known opinions concerning 

 the various I'pecics of luminoui rays; t'r.e 

 colouis'relultlng from a iT)ix.tu;e cf fcve- 

 ral of thefe rays taken at diffe'ert pans 

 of the folar fpefinim, and among otheri 

 the very lemarkable cafe, where the rays 

 are fo chofen, that their union produces 

 on the or£;an of fight the fenfation of 

 whitenefs, even it t-.vo forts of rays only 

 be employ eil. 



If wc would romprehend whit jisfHrs in 

 the firing ol co!oiirs, it is indilpeniable in 

 the fir!t place to obtsjn a f.'.niiliar iicquiunt- 

 ace with the fliades coirpofej of feveial 

 finiple lays, to acquire precife idias of 

 b'atk and white, ^and of the complication 

 thefe introHuce into coloured eppearances ; 

 and more efpccially ti underlland tie re- 

 lation of colours, which, taken two and 

 two in a certain order, are capable of 

 forming by their union white or any cth;r 

 compound tint. 



Two colouis having this kind of rela- 

 tion to each other aro-. reciprocally teimed 

 complimentary coloins. One of t''e!e be- 

 in; given, the oher may be <ieteimiiied 

 Tvit 1 ni re or l^f* piecifion by various 

 mo.Jes of expeiinieiit, .calculation, &c. ; 

 an I the cjnfideration ot them applies very 

 uftfully to a great numlier of cafes. 



Tiie anth'.r next ptgcstds to contrafts, 



that is, the effeft of the iimultaneous vi- 

 fion cf two I'ubltances differently coloured, 

 when brought near together under certain 

 circum(}ances. Contralf, then, is here a 

 comparifon, from wliich refults the fenti- 

 ment of a certain difference, great or 

 fmall. Colours exhibited by contraft- are 

 always cnformable to the tint that would 

 be obtained by abffrafting from the co- 

 lour proper to one of the fubftances the 

 rays anilogous to the colour of the other. 

 Thus if on red papsr a flip painted 

 orange-colour be placrd, the latter will 

 appear nearly yeilow, but the fame fti ip 

 placed on yeilow p.iper will appear nearly 

 red. It it be i laccd on violet-paper it 

 will refume a yell wifti tint ; and on green 

 paper it will appear red, but in a different 

 degree. 



The explanation of thefe ir.ftances by 

 the rule propolvd is eafy, if we fuppol'e 

 the orange-colour of the little ftrip to be 

 coinpounded of all the rays except blue, 

 which is commonly the cafe. A multi- 

 tu.lc of combinations of colours thus 

 placed upon one another bring out tiie co- 

 lour o{ confr:iit indicated by the rule laid 

 d )vvn ; but theie are feverai ciicumllances 

 that lender t!ie efTeif m re (triking, or mo- 

 dify the relult. Sometiines it depends on 

 the decree of li^ht by which the colours 

 are obfervtd, and fomitiines upon the 

 manner in which it enters thr-eye. 



The colours of contraft will appear 

 likewife with greater vividnefs after they 

 have been obieived a few moirents, or if 

 the colouied fubflarccs be fliaken a little, 

 lb that ihey may pafj flowly over the re- 

 tina. If feems as if a certain fatigue of 

 the eye, eitner iLilantantoufiy with regard 

 fo the interfity of light, or more flowly 

 by a prolonged vificn, concurred to pro- 

 duce ilie appearances in qucftion. But 

 an excefiive fatigue of the organ would 

 produce a degeneration of the colours be- 

 longing to another mode. 



Pkl. Prieur propofes a new method of 

 rendering the colours of contraft very fen- 

 fible. This tnethod confifts in the obfer- 

 ver being in a roin with a good light, and 

 placing :;gainft the window the coloured 

 papers on which he means to obferve the 

 contrafts in the manner above-mentioned. 

 The colouied paper ferving as the ground 

 will then poffcfs a degree of lemi tranlpa- 

 rency ; while the tittle flip of a difrlrent 

 colour placed upon it is more opake, and 

 in the fliade, on account cf the double 

 thicknefs of paper. Thus the colour pro- 

 duced by the comraft is rendered inucti 

 more lirlking. 



From 



