HERSCIIEL ON COLOURED RINGS. 125 



Sixth Method. Upon a 7 feet concave metalline mirror I 

 placed the double convex 20-inch lens, and had a very line 

 6et of ring's. 



Generalization. With these two last methods, whatever Gth. Lens in 

 may be the radius of the concavity of the subjacent surface, collcave meta ^ 

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

 cumbent glass; and whatever may be the figure of the up- 

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

 there will be produced concentric rings. The figure of the 

 iow est surface of the subjacent glass may also be varied at 

 pleasure, and still concentric rings will be obtained. 



II. Of seeing Rings by Transmission. 



The great variety of the different combinations of these Rino-s by trans- 

 differently figured glasses and mirrors will still admit of far- mission, 

 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 ob- 

 servation, where he saw these rings by transmission, in con- 

 sequence of which I have again multiplied and varied the 

 method of producing them that way, as follows: 



First Method. On a slip of plain glass highly polished on lst met ]-, 0(1 



both sides place the same double convex lens of 2(i-inches, Double convex 



which had already been used when the rings were seen by ^ ns on plain 



& J glass. 



reflection. Take them both up together and hold them 



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

 tric 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 of spectacles 

 -with glasses of 5, (i, or 7 inches focus, to magnify the rings 

 in order to see them more readily. 



Second Method. Tt would be easy to construct an appara- od. xi, e same 

 tusfor viewing the rings by transmisson fitted with a proper with tne Kf&i 

 eve-glass; but other methods of effecting the same purpose ^'"w! ^"^ 

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

 rings be laid upon a hollow stand, a candle placed at a pro- 

 per 



