312 L'ttmui.—J^cwicnhn Doflriue of Trart/m'JpiU tind RefeB'toiu 



with this new mixture certain trays are filletf, which are one metre (39^ inches) deep, and 

 eight decimetres {314^ inche>) wide. When the fermentatioa which takes place for the 

 third time has given the parte a confiderably deep blue colour, chnlk or powdered marble 

 i« added, and the whole is well and perfeftly mixed. TIils laft addition is made, not to im- 

 prove the quality of the blue, but to add weight. It is merely an affair of profit. The blue 

 thus prepared is put into iron moulds 32 centimetres long and 22 fquare at the end 

 (i| inch by x-^ of an inch). The moulded pieces are then placed upon deal planks, in welU 

 aJred lofts, to dry ; at*ter which they are packed in cafks for fale. 



The Hollanders made a fecret of this procefs: and in order to miflead, they have publifh- 

 ed, that the blue was made with rags coloured by the plant turnfol ; whence it has obtained 

 its denomination *. We may derive much profit by carrying this difcovery into praclice. 



VIII. 



Experiment! and Remarij on certain Ranges of Colours hitJierto unohferved, ivhich are productd 

 by the relative Pofition of plain Glajfes -with regard to each ether. {W, N.) 



T. 



HOSE ranges of colours which are afForded by the refleflion and tranfmiffion of light 

 through thin tranfparent plates, have been an obje£t of much attention to philofophers, evet 

 Cnce the experiments made by Robert Hooke and Sir Ifaac Newton. The latter of thefe 

 philofophers endeavoured to generalife the fads by a ilatement, which, becaufe in part hy- 

 pothetical, has been treated with contempt by fomc eminent men, though it has been re- 

 ferred to in moft difquifitions on thefe phenomena. When a convex lens is applied to a 

 plain glafs, it is well known that coloured circles furround the place of contaft j and as 

 thefe circles are found to vary in their dimenfions, the nearer the glafles lie with refped to 

 each other, whether by means of the curvature or of preffure, it has been concluded thafr 

 tlie effeds are caufed or governed by the diftances of the furfaces. From various confidera- 

 tions Sir Ifaac was led to conclude, that the rays of light are themfelves poffefled of a pro- 

 perty, by which, in certain equidiftant points of their length, they are difpofed to entCF 

 tranfparent bodies, and in certain other points, intermediate between thefe, they are dif- 

 p6fed to be refleded. From this aflumption he deduced, that if a ray of light pafled 

 through the firft furface of a medium, it would be either tranfmitted or teflcfled at the 

 fecond furface, accordingly as the diftance might happen to coincide with a point of tranf- 

 miffion or refle£lion. According to- this doftrine, it muft follow that the interval will 

 govern the effect, not in confequence of its own magnitude, but of the precife number of 

 »neafures, or, as he calls them, fits of tranfmiffion or refledion it may contain. 



Thus, for example, if we fuppofe a ray of indefinite length to be divided into equal parts,, 

 and imagine certain marks of divifion to fubfift ; if the ray pafs, through the firft furface of a 

 medium exadly at one of thefe marks, it will be tranfmitted through any furface, however 

 diftant, which (hall be remote from the firft, either nearly or accurately fome precife num- 

 ber of parts ; beeaufe the fits of tranfmiffion are refpedively at thofe points or marks :. and 



•Englilh writers have ufed this denomination. But the dry-falters, « dealers in drugs, diftingui/h thefe 



M&ih by the name of litmus. 



