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 in. (= 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 of 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. — 0.800 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 in. (= 3.1750 mm.). 



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

 long, not to exceed 0.759 in. (= 19.2784 mm.) in diameter. 



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

 3.1750 mm.). 



u Limits 

 " Nose-piece: 



" Core Diameter of Thread (A) not to exceed 0.7674 in. (= 194-91 8* 

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



" Full Diameter of Thread (B) not to exceed 0.S03 in. (= 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.). 



