IIERSCHEL ON COLOURED RINCS. 1^9 



green one, while at the same moment the green centre was, 

 turned into orange. With a different contact I have had the 

 primary set with a blue centre and the secondary with a deep 

 yellow one; and by bringing the second and third shadows 

 alternately over the primary set, the blue centre was changed 

 to a yellow, and the yellow centre to a blue one ; and all the 

 rings of both sets had their share in the transformation of co- 

 lour and size. 



If there are three sets of rings, and the primary set has a 

 black centre, the other two will have a white one; and when 

 the lowest shadow is made to fall on the third set, the central 

 colour of all the three sets will be suddenly changed, the first 

 from black to white, the other two from white to black. 



A full explanation of these changes, which at first sight 

 have the appearance of a magical delusion, will be found in a 

 future article. 



XVI. Of the Course of the Rays by which different Sets 

 of Rings are seen. 



In order to determine the course of the rays, which give Determination 



the rings both by reflection and bv transmission, we should °. J!, course 

 c> " j ine ru. j s. 



begin from the place whence the light proceeds that forms 

 them. In PI. IV, fig. 1 , we have a plano-convex lens laid upon 

 three slips of glass, under which a metalline mirror is placed. 

 An incident ray 1, 2, is transmitted, through the first and 

 and second surface of the lens, and comes to the point of 

 contact at 3. Here the rings are formed, and are both re- 

 flected and transmitted : they are reflected from the upper 

 surface of the first slip, and pass from 3 to the eye at 4: 

 they are also transmitted through the first slip of glass from 

 3 to 5 ; and at 5 they are again both reflected and transmit- 

 ted ; reflected from 5 to 6, and transmitted from 5 to 7 ; from 

 7 they are reflected to 8, and transmitted to 9; and lastly 

 they are reflected from 9 to 10. And thus four complete sets 

 of rings will be seen at 4, 6, 8, and JO. 



The most convenient way of viewing the same rings by 

 transmission is that, which has been mentioned in thesecoud 

 article of this paper, when light is conveyed upwards by re- 

 flection. In figure 2, consisting of the same arrangement 

 of glasses as before, the light by which the rings are to be 



seen 



