The Boyal Microscopical Society's Standard Screw-Thread. 391 
They are also having made a series of dies of the nominal inside 
diameter on the top of the thread of O’ 7611 in., which will not vary 
more than from 0*7601 to 0*7621. To test this the Council has 
ordered a Whitworth plug and ring, size 0*7596 in. diameter, and 
no die will be allowed to be stamped with the Society’s stamp unless 
it will pass easily over the 0*7596 in. plug and will not pass over 
the 0 * 7 626 in. plug. 
These taps and dies will be for sale almost immediately, at cost 
price, 21 15s. for each pair of tap and dies, and it is earnestly 
requested that every maker of Microscopes will possess himself of a 
pair of these sizing gauges. 
The Council believe that at such time as these sizing taps and 
dies have come into universal use the standard screw-thread will have 
been put upon a permanent basis, and complete interchangeability of 
all object-glasses will have been established. 
Specification of 
The Royal Microscopical Society Standard Screw. 
Thread . — Whitworth screw, i.e. a Y-shaped thread, sides of thread 
inclined at an angle of 55° to each other, one-sixth of the Y depth 
being rounded off at the top and the bottom of the thread. 
2 e 2 
