Jmperfeci'omfrom Colour in Speculums, 153 



rings were exa£lly the fame with thofe of thick plates (to ufe Newton's expreffion) ; for 

 the c»at of gums is, when thin, pretty tranfparent, as may be feen by laying one oa glafs 

 pJatcs. 



But this coat is extremely thin, and cannot exceed the 2octh part of an inch ; fo that the 

 colours of thick plates are in fa£l the very fame with thofe of thin plates, except that the 

 two kinds are made by different fized plates. We cannot, therefore, diftinguifh them, any 

 more than we do the fpe£truni made by a prifm whofe angle is 90" from that made by one 

 whofe angle is 20°. This kind of colours is not the only one I have obferved of nearly the 

 fame kind with thofe of plates ; we fhall prefently fee another much more curious and re- 

 markable. 



III. 



IN reflecting on the obfervations and concluiions contained in my former paper, feveral 

 confequences feemed to follow which appeared fo new and uncommon, that I began to 

 doubt a little the truth of the premifes ; but at any rate was refolved to examine more mi- 

 nutely how far thefe inferences might be ccnfiftent with fa<5i : and I am happy in being able 

 to announce the completenefs of that confiftency, even beyond my expeftations. The chief 

 confequences were the following : 



1. That a fpeculum fhould produce, by flexion and reflexion, colours in its reflected light 

 wherever it has the leaft fcratch or imperfection on its furface. 



2. That on great inclinations to the incident rays all fpecula, however pure and highly 

 poliflied, fhould produce colours by flexion. 



3. That they {hould alfo in the fame cafe produce colours by reflexion. 



4. That lenfes, having the finalleft imperfections, fhould produce by flexion colours in 

 tlieir refradted light. 



5. That there fhould be many more than three, or even four fringes by flexion, invifibic 

 to the naked eye. And, '' 



6. That Iceland cfyftal fliould have fome peculiarities with refpeCt to flexion and re- 

 flexion ; or if not, that fome information fliould be acquired concerning its fingular proper- 

 ties refpedling refra£tion. * 



The manner in which the firft of thefe proportions is demonflrated a priori, is evident 

 from the 4th figure, where CD is the refleding furface, voa concavity bearing a fmall ratio 

 to CD, Ao and AB rays proceeding to CD. The one, AB, will be feparated into Br red, 

 and Bv violet, by deflexion from o, and will be refledted to r' v', forming there the fringes. 

 The other, Ao, being reflecSted, will be feparated into Bx and By, by deflexion from v, 

 forming other fringes, xy, on the fide of vo's fhadow oppofite to rV. Alfo when vo is con- 

 vex inftead of concave, the like fringes will be produced by the rays being defle£tcd in paflT- 

 ing by its fides. Laftly, when vo is a poliflied ftreak, images by reflexion will be produced, as 

 defcribed Phil. Tranf. for 1 796, p. 269. (Philof. J. i. 593.) The lame paflTage will alfo fliew the 

 reafon why, on great incUnations, colours by reflexion fliould be produced. And the fecond 

 propofition, with refpe£t to flexion, follows from what was demonftrated in this paper (p. 149 

 and 150) ; it being that cafe where the rays either leave or fall on the fpeculum at fuch an 

 inclination as to come only within the fphere of inflexion, without being deflc(9ted. The 



Vol. II — ^JtjXY 1798. X fburtk 



