the Cause of coloured cmcentrlc 'Rings, 77 



provided it be greater than that of the convexity of the in- 

 cumbent glass ; and whatever may be the figure of the upper 

 surface of the lenses that are placed upon the former, there 

 will be produced concentric rings. The figure of the lowest 

 surface of the subjacent glass may also be varied at pleasure,: 

 and still co;icentric rings will be obtained. 



,^ II. Of seeing Rings ly Transmission, 



The great variety of the different combinations of these 

 differently figured glasses and mirrors will still admit of fur- 

 ther addition, by using a different way of viewing the rings. 

 Hitherto^ the arrangement of the apparatus has been such as 

 to make them visible only by reflection, which is evident, 

 because all the experiments that have been pointed out may 

 be made by the light of a candle placed so that the angle of 

 incidence and of reflection towards the eye of the observer 

 may be equal. But Sir I. Newton has given us also an 

 observation where he saw these rings by transmission, i» 

 consequence of which I have again multiplied and varied the 

 method of producing them that way, as follows : 



First Molkod, On a slip of plain glass highly polished on 

 both sides place the same double convex lens of 26 inches, 

 which had already been used when the rings were seen by 

 reflection. Take them both up together and hold thein 

 against the light of a window, in which position the con- 

 centric rings will be seen with great ease by transmitted 

 light. But as the use of an eye-glass will not be convenient 

 in this situation, it will be necessary to put on a pair oi 

 spectacles with glasses of 5, 6, or 7 inches focus, to mag- 

 nify the rings in order to see them more readily. 



Second Method, It would be easy to_ construct an appara- 

 tus for viewing the rings by transmission fitted witii a pro- 

 per eye-glass ; but other methods of effecting the same pur- 

 pose are preferable. Thus, if the two glasses that are to 

 give the rings be laid upon a hollow stand, a candle placed 

 at a proper -angle and distance under them will show the 

 rings conveniently by transmitted light, while the observer 

 and the apparatus remain in the same situation as if they 

 were to be seen by reflection. 



i Third 



