230 



OBJECTIVE SCREW THREAD. 



(Fig. 29.) 



The question of standardization of the Objective Screw Thread was 

 first discussed by the Microscopical Society in 1857,* and the first sizing 

 tools were issued in 1858. 



In 1806| the Council of the Royal Microscopical Society issued 

 another Report, and drew up a specification defining the limits of varia- 

 tion allowable from the original standard screw thread. 



Difficulties having arisen in connexion with the testing and adjusting 

 of the sizing tools supplied by the Society,! the Council in 1911 

 appointed a Gauges Committee to look into the question of obtaining 

 and testing further tools, and they now have pleasure in informing 

 Fellows of the Society that an arrangement has been made with the 

 Director of the National Physical Laboratory whereby the standard 

 gauges of the Society have been deposited at the National Physical 

 Laboratory. The Council has also arranged for the issue of new ob- 

 jective 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*3900 mm.). 



Die for sizing Objective : core diameter of thread between * 759C 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. 



Pitch.— 36 to the inch. 



Form. — "Whitworth screw, i.e. a Y-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. 



* Trans, of the Microscopical Society, 1858, p. 39 ; 1859, p. 92. 

 t Trans, of the Royal Microscopical Society, 1896, pp. 389, 487. 

 \ In this connexion the paper published in the Trans. R.M.S., 1911, p. 175, 

 is interesting. 



