VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 203 



same kind with those of plates ; we shall presently see another much more curious 

 and remarkahle. 



III. In reflecting on the observations and conclusions contained in my former paper, 

 several consequences seemed to follow, which appeared so new and uncommon, 

 that I began to doubt a little the truth of the premises ; but at any rate was re- 

 solved to examine more minutely how far these inferences might be consistent with 

 fact : and I am happy in being able to announce the completeness of that con- 

 sistency, even beyond my expectations. The chief consequences were the follow- 

 ing, l. That a speculum should produce, by flexion and reflexion, colours in its 

 reflected light wherever it has the least scratch or imperfection on its surface. 

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

 highly polished, should produce colours by flexion. 3. That they should also in 

 the same case produce colours by reflexion. 4. That lenses, having the smallest 

 imperfections, should produce by flexion colours in their refracted light. 5. That 

 there should be many more than 3, or even 4 fringes by flexion, invisible to the 

 naked eye. And, 6\ That Iceland crystal should have some peculiarities with re- 

 spect to flexion and reflexion ; or if not, that some information should be acquired 

 concerning its singular properties respecting refraction. 



The manner in which the first of these propositions is demonstrated a priori, is 

 evident from fig. S, where cd is the reflecting surface, vo a concavity bearing a 

 small ratio to cd, ao and ab rays proceeding to cd. The one, ab, will be sepa- 

 rated into Br red, and bv violet, by deflexion from o, and will be reflected to rV, 

 forming there the fringes. The other, ao, being reflected, will be separated into 

 bx and By, by deflexion from v, forming other fringes, xy, on the side of vo's 

 shadow opposite to rV. Also when vo is convex, instead of concave, the like 

 fringes will be produced by the rays being deflected in passing by its sides. Lastly, 

 when vo is a polished streak, images by reflexion will be produced, as described in 

 my former paper for 179^* The same passage will also show the reason why, on 

 great inclinations, colours by reflexion should be produced. And the 2d propo- 

 sition, with respect to flexion, follows from what has been demonstrated in this 

 paper, it being that case where the rays either leave or fall on the speculum at such 

 an inclination, as to come only within the sphere of inflexion, without being de- 

 flected. The 4th proposition is merely a simple case of flexion. And the last 2 

 require no illustration. I shall now relate how I inquired into the truth of these 

 things a posteriori. 



Observ. 1. Looking at a plane glass mirror exposed to the sun's light, I observed 

 that up and down its surface there were minute scratches, called hairs by workmen, 

 and that each of these reflected a bright colour, some red, others green, and others 

 blue. On moving the mirror to a different inclination, or my eye to a different 

 position with respect to the mirror, I saw the species of the colours change; the red, 

 for instance, became green, and the green blue. I applied my eye close to the 

 mirror, and received on it the light reflected from one hair. I observed several dis- 



D D 2 



