MICROGRAPHUAY 52° 
all turn’dinto a litharge;. for that colour which covers the top, being 
feum’d-or fhov'd afide, appears to be nothing elfe but a litharge or 
vitrify d Lead. ae Sti t 495 vitten  — oe 
‘Thisis obfervable alfo infome fort , on Bra&, Copper, Silver, Gold, 
Tin, but: is moft confpicuous in Lead: all thofe Colours that. cover the. 
farface of the Metal being nothing elfe., but a very. thin vitrifi'd part 
of the heated Metals) eyrs'do tiem : her 
The other Inftance we have, is in Animal bodies, .as.in Pearls, Mother 
of Pearl-fhels, Oyfter-fhels, and .almoft all other kinds. of ftony thels 
whatfoever.. Thishave I alfo fometimes with pleafure obferv'd even 
in Mufclesand Tendons... Further, ‘if you take any glutinous fubftance 
and runit exceedingly thin upon the furface of a{mooth glafs or a po- | 
lifht metaline: body, you fhall find the. like effeG&s produced: and in | 
general, wherefoever you meet with a tranfparent body thin. enough, | 
that is terminated by reflecting bodies of differing refractions from it, — 
there will be a production of thefe pleafing and lovely colours... | 
- Norisitneceflary, that the two terminating Bodies fhould be both of 
the fame kind, as may appear by the wtrified Lamina on Steel, Lead, and 
other Metals,one furface of which Lamine is contiguousto the furface of 
the Metal, the other to that of the Air... . , ae 
Nor isit neceflary, that thefe colour'd Leming fhould be of an:even 
thicknefs, that is, fhould have their edges and middles of equal thicknefs, 
as ina Looking-glafs-plate, which circumftance.is only requifite to make 
the Plate appear all of the fame colour ; but they may refemble a Lens, 
that is, have their middles thicker then their edges; or elfea double con- 
cave, that is, be thinner in the middle then at the edges 5 in both which. 
cafes there will be various coloured rings or Serer ginicitierne confecu- 
tions or orders of Colours 5 the order of the firft from the middle out- 
wards being Red, Yellow, Green, Blew, cs And the latter quite con- 
trary. : 
“ "Bat further, it is altogether necefiary,, that the Plate, in the places 
where the Colours appear, fhould be of « determinate thicknefs :. Firft, It 
muft not be more then fuch a thicknefs, for when the Plate isincreafed to 
fach athicknef&; the Colours ceafe 5 and_befideés, I have feen in athin 
piece of Atfcovy-glafs, where the two ehds of two Plates. which appear- 
ing both fingle; exhibited two diftin& and differing Colours 5 but 
in that place where they were united, and conftituted one double Plate 
(asI may call it) they appeared tranfparent. and colourlefs. Nor, Se- 
condly , may the Plates be shiver then fuch.a determinate cize 5 for we 
alwayes find, that the very outmoft Rim of thefe flaws is terminated in — 
a white and coloutlef§ Rings. re, - 
Further, in this Production of Coloursthere istio need of a determi- 
nate Light of fucha bignefs and no more ; ‘nor of a determinate pofition 
of that Light.that it fhould be on thisfide,and not on that fide; nor of a 
terminating fhadow, asin the Prifme, and Rainbow; or Water-ball «, for 
we find, that the Light inthe open Air, either in or out of theSun-beams; 
and within a Room, either from one or many Windows, produces ee 
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