Vticewmon C*hUrt preJuftd in flat Phtes of Glafs, 3 rj 



en the other hand, it will be refle(fted Trotn any furface whofe diftance is meafured by fome 

 f number of whole parts, together with half a part ; becaufc the fits of refleftion lie half-way 

 between the divifions. In this ftatement I have merely enunciated what are fuppofed to be 

 the fails, aird have not attended to his fuppofition of an etherial elaftic fluid, pervading 

 «11 fpace and thrown into undulations more rapid than the velocity of light itfelf. Neither 

 can I at prefent enter into any dlfcuffion concernit/g its probable value. My prefent obj/idV 

 is to relate a few experiments, which, at the fame time that they (hew the exiftence of 

 thefe fits at very confiderable intervals, appear to prove that the refle(!^ion and tranfmifiioii 

 of light in the fame medium is governed by other circumflances as well as by the diftance 

 between the confines or furfaces. Thefe experiments lead to many fpeculations and ex- 

 tended paths of enquiry. It has long been my intention to multiply and repeat them, in 

 order to difcover the laws upon which they depend : but other avocations have already de-^ 

 layed the accomplilhment of this purpofe for more than feven years ; for which reafon, I 

 hope that my defire to fee the objed purfued by others may operate as an apology for the 

 imperfe£l ftate of the refearch, of which I here prefent the commencemen^ 



The experiments of the Abbe Mazeas on colours produced by applying flat plates of glafs 

 to each other, are related at full length in Prieftley's Hiftory of Light and Colours, p. 499. 

 This philofopher found, that rubbing the glafles together caufed the colours to appear at the 

 fame time that adhefion took place between them, and Mufchenbroeck found thatlenfes of 

 long focus do not afford colours after having been, laid by for a length of time unlefs they 

 be wafhed and wiped. One of the moft remarkable circumftances in the experiments of 

 Mazeas was, that the colours of his flat plates are moveable by a very flight increafe of 

 temperature; which produces an efte£l fimilar to what would have arifen from removing 

 the glafles further afunder, or diminifliing the preflure which might have been applied to 

 them. Beccaria was, I believe, the firft who obferved that thefe colours may be produced 

 by fuperinducing an eledric charge on the external furfaces of two plates, which are by 

 this means made to adhere. But in all the obfervations I have met with, whether made by 

 Prieftley in his Hiftory, or by any other writer, thefe powers have been fuppofed to operate 

 by increafing or diminifliing the diftance between the furfaces. The following fadts will 

 fliew that this conclufion requires to be modified. 



In the year 179 1 I cut a plate of very clear glafs into portions, which were intended to- 

 be ufed to defend a veflel of quickfilver from the agitation of the air. Its colour, whea- 

 viewed edgewife, was a very light green, and it had been carefully ground by an optician^ 

 ■with its fides truly parallel to each other. The thicknefs was twelve hundredths of aa 

 inch. A piece 3,2 inches long and 2,4 \ii'ide was laid upon another larger piece, both 

 having been previoufly wiped. The refult was, that faint colours appeared in rows about 

 fix or feven in number. Preflure appeared to alter thefe very little in pofition 5 but it pro- 

 duced other more vivid colours, which were much more moveable, and crofled the former 

 without afFe£ling them. The fcarcely moveable ranges were very little diftuxbed by the heat 

 of the finger applied againft the undermoft glafs; though this application was fuSicient to 

 produce a very great alteration in the figure of the other ranges^ When the upper glafs. 

 was lightly placed upon the lower, the faint and fcarcely moveable ranges appeared alone,, 

 and the glafles had very little adhefion ; fo that the uppermoft Aided about on the larger 

 jilate beneath, without feeming to carry the ranges with it, but arrived at other ranges^ 



whichi 



