1?8 Experiments for investigating, &c. 



the perfect part of the lens was put a small lump of wax^ 

 soft enough to allow a gentle pressure to bring the point of 

 contact towards the fractured edge, and to keep it there. 

 In this arrangement it has already been shown that there are 

 two different ways of seeing two sets of rings : by the rays 

 ], 2, 3, we see a primary set; and by 1, 2, 4, 5, the secon- 

 dary set belonging to it : by the rays 6, 7, 2, 3, we' see a 

 different primaiy set ; and by 6, 7, 2, 4, 5, we see its secon- 

 dary one. That this theory is well founded has already been 

 proved ; but if we should have a doubt remaining, the in- 

 terposition of any small opaque object upon the looking- 

 glass near the fracture will instantly stop the latter two sets 

 of rings, and show the alternate colours of the two sets that 

 will then be seen by the rays 1, 2, 3, and 1, 2, 4, 5. Re- 

 move in the next place the stop from the looking-glass, and 

 bring the second shadow of the penknife over the primary 

 set, and there will then only remain the two sets of rings 

 formed by incident rays which come from 6, and which 

 have never passed through the upper surface of the lens. 

 Now, as both sets of rings in this case are completely form- 

 ed by rays transmitted upwards from the coated part of the 

 looking-glass without passing through the first surface of 

 the incumbent lens, the proof that the modifying power of 

 that surface is not required to the formation of the rings is 

 established. 



It can hardly be supposed that the first surface of the 

 lens should have any concern in the formation of the rings 

 when the rays are reflected from the looking-glass towards 

 the eye; but the same experiment, that has proved that this 

 surface was not required to be used with incident rays, will 

 show that we may do without it when they are on their re- 

 turn. We need only invert the fractured lens, as in figure 

 3 2, when either the rays 1, 2, 4. 5, or 6, 7, 2, 4, 5, will 

 convey the image of the rings after their formation to the 

 eye without passing through any part of the lens. 



[To be continued.] 



xxvr. Ob- 



