202 Dr. Herschel’s Experiments for investigating 
center of the secondary one will be green ; or if the former 
happens to be green, the latter will be red or orange. At the 
same time there will be a similar alternation in the size of rings ; 
for the white rings in one set will be of the diameter of the 
black in the other ; or the orange rings of the former will be 
of equal magnitude with the green of the latter. 
When three sets of rings are to be seen, the second and third 
sets will be alike in colour and size, but alternate in both par- 
ticulars with the primary set. 
The same thing will happen when four sets are visible ; for 
all the sets that are formed from the primary one will resem- 
ble each other, but will be alternate in the colour and dimen- 
sions of their rings with those of the primary set. 
XV. Of the sudden Change of the Size and Colour of the Rings 
in different Sets. 
When two sets of rings are viewed which are dependent 
upon each other, the colour of their centers and of all the rings 
in each set, may be made to undergo a sudden change by the 
approach of the shadow of the point of a penknife or other 
opaque slender body. To view this phenomenon properly, 
let a 1 6 inch double convex lens be laid upon a piece of look- 
ing- glass, and when the contact between them has been made 
to give the primary set with a black center^ that of the secon- 
dary will be white. To keep the lens in this contact, a pretty 
heavy plate of lead with a circular hole in it of nearly the dia- 
meter of the lens should be laid upon it. The margin of the 
hole must be tapering, that no obstruction may be made to 
either the incident or reflected light. When this is properly 
arranged, bring the third shadow of the penknife upon the 
