1 $8 Experiments for investigaihtg 



XXVI. Of the Action ef the third Snrfoce, 

 When a double convex lens is laid upon a plain metalline 

 mirror that happens to have an emery scratch in its surface, 

 we see it as a black line under the rings that arc formed over 

 them. This shows, that when a defect from want of polish 

 has not a power to reflect light in an ii regular manner, it 

 cannot distort the rings that are formed upon it. 



When I laid fi good 21 -feet object glass upon a plain slip 

 that had some defects in its surface, the rings, in every part 

 of the object glass that was brought over them, were always' 

 disfigured ; which proves that a reflection from a defective 

 third surface has a power of forming distorted rings, and 

 that consequently a reflection from one that is perfect must 

 have a power of forming rings without distortion, when it 

 id combined with a proper second surface. 



When the defective slip of glass, with a perfect lens upon 

 it, was placed uppn a metalline mirror, T saw the secondary 

 stt affected by distortions of the rings that were perfectly 

 like those in the primary set; which proves that a polished 

 defect in the third surface will give modifications to the rays, 

 that form the rings by transmission as well as by reflection. 



XXVII. The Colour of the reflecting mid transmitting Sur- 

 faces is of no consequence. 



I laid seven 54-inch double convex lenses upon sevea 

 coloured pieces of plain glass. The colours of the glasses 

 were those which are given by a prism, namely, violet, 

 indigo, blue, green, yellow, orange, and r^d. The rings 

 yeflcctcd from each of these glasses were in every respect 

 alike^ at least so far that I coxdd have a black, a white, a 

 red, an orange, a yellow, a green, or a blue centre with 

 every one of them, according to the degree of pressure I 

 used. The lenses being very transparent, it may be admit- 

 ted that the colours of the glasses seen through them would 

 m some degree mix with the colours of the rings; but the 

 action of the cause that gives the rings was not in the least 

 affected by that circumstance. 



I saw the rings also by direct transmission through all 



the coloured glasses except a dark red, which stopped sq 



. "-"/y -. much 



