128 HERSCHEL ON COLOURED RINGS. 



formed, as has been explained, we shall not find it difficult 

 to see them iu these situations, 



1st method. First Method. Upon a well polished piece of good look- 



ou e eon vex j^ g] ass ] av down a double convex lens of about i>0 inches 



glass: focus. When the eye-glass has been adjusted as equal for 



seeing one set of rings, make the shadow of the penknife 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 shadow will, in its passage over the 

 surface, 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. 



Secondary set Then, if at the same time a secondary set of rings lias 



«tf ruigs. not yet been discovered, it will certainly be perceived when 



the second shadow of the penknife is brought upon the pri- 

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

 shadow, consisting of all the three shadows united, may 

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

 at leisure and in perfection. I>ut this compound shadow 

 should be taken no farther 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, 



\ievred alter- » , 



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

 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. 

 The rinws made Besides the use of the shadows, there is another way to 

 ■visible by set- make rings visible when they cannot be easily perceived, 



mrJiiun!" 1 ^ which is to take hold ° f the lens wit]l b ° th hands > to P ress 

 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 perceived 

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

 be generally detected together. 



_.',..■, It will also contribute much to faciliate the method of 



Thelicht . .... 



should be cbli- seeing two sets of rings, rf we receive the light in a more 



*l ue - oblique angle of incidence, such as 40, 50, or even GO de- 



grees 



