Ch. Ill] ROYAL MICROSCOPICAL SOCIETY STANDARDS 105 



objective screw sizing taps and dies, which have been tested and 

 passed by the N.P.L. and are within the following limits: 



" Tap for sizing Nose-pieces: full diameter between 0.800 in. 

 ( = 20.3198 mm.) and 0.803 m - (— 20.3960 mm.). 



" Die for sizing Objective: core diameter of thread between 0.7596 

 in. (= 19.2937 mm.) and 0.7626 in. (= 19.3699 mm.). 



" A certificate *>f accuracy is issued with each tap and die. These 

 sizing tools are now on sale, and may be obtained by application to 

 the Secretaries of the Royal Microscopical Society. 



" The standard specification for the objective thread has not been 

 altered, and is as follows: 



"Specification of the Royal Microscopical Society 

 Standard Screw Thread for Objectives 



" Metrical Measurements in Brackets 





Diameter. — 0.S00 in. [20.3198 mm.]. 

 Pitch. — 36 to the inch [14.17 to the cm.]- 



" Form. — Whitworth screw, i.e. a V-shaped thread, sides of thread 

 inclined at an angle of 55 to each other, one-sixth of the V depth 

 being rounded off at the top and the bottom of the thread (fig. 64). 



" Length of Thread on Objective, 0.125 m - ( = 3- I 75° mm.). 



"Plain Fitting above Thread of Objective, 0.1 in. (= 2.5400 mm.) 

 long, not to exceed 0.759 m - ( = 19-2784 mm.) in diameter. 



" Length of Screw of Nose-piece to be not less than 0.125 in. ( = 

 3.1750 mm.). 



"Limits 

 " Nose-piece: 



" Core Diameter of Thread (A) not to exceed 0.7674 in. (= 19.4918 

 mm.), or be less than 0.7644 in. (= 19.4156 mm.). 



" Full Diameter of Thread (B) not to exceed 0.803 m - ( = 20.3960 

 mm.), or be less than 0.800 in. (= 20.3198 mm.). 

 " Objective: — 



" Full Diameter of Thread (C) at top of thread not to exceed 0.7982 

 in. (= 20.2741 mm.), or to be less than 0.7952 in. (20.1979 mm.). 



