’jq8 Dr. Herschel's Experiments for investigating 
generally 8, 9, or 10, very conveniently. By holding the eye 
in the most favourable situation I have often counted near 20, 
and the number of them is generally lost when they grow too 
narrow and minute to be perceived, so that we can never be 
said fairly to have counted them to their full extent. In the 
second set I have seen as many as in the first, and they are 
full as bright. The third set, when it is seen by a metalline 
mirror under two slips, will be brighter than the second, and 
almost as bright as the first: I have easily counted 7, 8, and 9 
rings. 
XI. Of the Effect of Pressure on the Colour of the Rings. 
When a double convex object glass of 14 or 15 feet focus 
is laid on a plain slip of glass, the first colours that make their 
faintest appearance will be red surrounded by green; the 
smallest pressure will turn the center into green surrounded 
by red : an additional pressure will give a red center again, 
and so on till there have been so many successive alterations 
as to give us six or seven times a red center, after v/hich the 
greatest pressure will only produce a very large black one 
surrounded by white. 
When the rings are seen by transmission, the colours are 
in the same manner subject to a gradual alternate change 
occasioned by pressure ; but when that is carried to its full 
extent, the center of the rings will be a large white spot sur- 
rounded by black. 
The succession and addition of the other prismatic colours 
after the first or second change, in both cases is extremely 
beautiful; but as the experiment may be so easily made, a 
