80 Experiments for investigating 



seeing one set of rings, make the shadow of the peilknife, 

 in the order which has been described, pass over the lens; 

 then, as it sometimes happens in this arrangement that no 

 rings are easily to be seen, the shado\^ will, in its passage 

 over the suiface, show where they are situated. When a 

 set of them is perceived, which is generally the primary one, 

 bring the third shadow of the penknife over it, in which 

 situation it will be seen to the greatest advantage. 



Then, if at the same time a secondary set of rings has not 

 yet been discovered, it will certainly be perceived when the 

 second shadow of the penknife is brought upon the primary 

 set. As soon as it has been found out, the compound sha- 

 dow_, consisting of all the three shadows-united, may (hen 

 be thrown upon this secondary set, in order to view it at 

 leisure and in perfection. But this compound shadow should 

 be taken no further from the point than is necessary to cover 

 it ; nor should the third shadow touch the primary set. The 

 two sets are so near together, that many of the rings of one 

 set intersect some of the other. 



When a sight of the secondary set has been once obtained, 

 it will be very easy to view it alternately with the primary 

 one by a slight motion of the penknife, so as to make the 

 third shadow of it go from one set to the other. 



Besides the use of the shadows, there is another way to 

 make rings visible when they cannot be easily perceived, 

 which is to take hold of the lens .vith both hands, to press 

 it alternately a little more with one than with the other; a 

 tilting motion, given to the lens in this manner, will move 

 the two sets of rings from side to side ; and as it is well 

 known that a faint object in motion may be sooner perceiv- 

 ed than when it is at rest, both sets of rings will by these . 

 means be generally detected together. 



It will also contribute much to facilitate the method of 

 seeing two sets of rings, if we receive the light in a more 

 oblique angle of incidence, such as 40, 50, or even 60 de- 

 grees. This will increase the distance between the centres 

 of the primary and secondary sets, and at the same time oc- 

 caaion a more copiou:* reflection of light. 



instead of a common looking-glass a convex glass mirror 



may 



