^^f Experiments /or investigating 



and that all the sets of rings we perceive are really reflected 

 or transmitted in the manner that has been pointed out ; 

 but as we have so many reflections and transmissions before 

 us, it will be necessary to confine these expressions to one 

 particular signification when they are applied to a set of 

 rings. 



When the centre of the rings is seen at the point of con- 

 tact, it is a primary set ; and I call it reflected, when the 

 rays which come to that point and form the rings undergo 

 an immediate reflection. But I call it transmitted, when 

 the rays after having formed the rings about the point of 

 contact are immediately transmitted. 



Th\is in figure 3 and 4 the rays a h c, d ef, give reflected 

 sets of rings j and the rays g h i, k I m, in figure 3 and 6, 

 give transmitted sets. 



In this denomination, no account is taken of the course 

 of the rays before they come to a, d,. g, k ; nor of what be- 

 comes of them after their arrival at c, f, i, m : they may 

 either come to those places or go from them by one or more 

 transmissions or reflections, as the case may require j .but our 

 donomination will relate only to their course immediately 

 after the formation of the rings between the glasses. 



The secondary and other dependent sets will also be called 

 reflected or transmitted by the same definition ; and as a set 

 of these rings formed originally by reflection may come to 

 the eye by one or more subsequent transmissions ; or being 

 formed by transmission, may at last be seen by a reflection 

 from some interposed surface, these subsequent transmis- 

 sions or reflections are to be regarded only as convenient 

 ways to get a good sight of them. 



With this definition in view, and with the assistance of a 

 principle which has already been proved by experiments, we 

 may explain sonie very intricate phaenomena; and the satis- 

 factory manner of accounting for them will establish the 

 truth of the theory relating to the course of rays that has 

 been described. 



The principle to which I refer is, that when the pressure 

 is such ^s to give a blapk centre to a set of rings seen by 



reflection^ 



