246 Mr. T. C. Porter on a Method 



of this second plate, A 8 , A 4 in fig. 2. If now the lower glass 

 plate be moved up till its upper surface is in contact with the 



Fig. 2. 



1 1 A 





1 I A, 





1 1 A. 





1 1 A, 





I 1 A 4 



lower surface of the upper plate, the two reflexions A 3 , A 4 will 

 be seen to move up with it till, when the two plates are in 

 contact, A 3 coincides with A 2 , and the appearance of the 

 images is that represented in fig. 3. It is evident that the 



Fig-. 3. 



: a, 



3 A 2 A 3 



] A 4 



middle image is caused solely by the light reflected from the 

 lower surface of the upper plate, and from the upper surface 

 of the lower plate. If we substitute for the two glass plates 

 the apparatus generally used for exhibiting Newton's rings, 

 we can in this simple way view the rings by light coming 

 from the two interior surfaces only, and thus completely free 

 from either light reflected from the upper surface of the 

 upper plate, or from the lower surface of the lower plate. 



Thus viewed, the central area appears of a velvety black 

 and the colours of the rings exceedingly brilliant. The whole 

 experiment can be easily projected, and the difference in the 

 appearance of the rings on the screen with and without the 

 slit is very striking. But the interest of the method does not 

 end here ; for besides affording an easy and obvious proof 

 that the rings are caused by two reflexions, one at each of 

 the two inner surfaces, and, under these circumstances, by 

 these two reflexions only, it also supplies a method of seeing and 

 distinguishing the interference-curves caused by light which 

 has undergone 1, 2, or 3 reflexions (forming the ring-system 

 usually seen, and named after Sir Isaac), and the curves 

 formed by the interference of rays which have suffered 4, 5, 

 and 6 reflexions or more. For if the reflexion of the slit in a 

 single glass plate be viewed more obliquely, as suggested 



