CH. II} LIGHTING AND FOCUSING 77 



In this last paper of four pages the matter 19 very carefully gone over and 

 full specifications of the new screw given. It conforms almost exactly with 

 the original standard adopted by the society, but means have been devised by 

 which it may be kept standard. 



This paper is of so much importance historically and practically that it de- 

 serves to appear in every work on the modern microscope. It is therefore 

 here repeated entire : 



FROM THE JOURNAL OF THE ROYAL MICROSCOPICAL SCCIETY 

 AUGUST, 1896 



" The Royal Microscopical Society's Standard Screw-Thread for Nose- 

 piece and Object-Glasses of Microscopes." 



" Being the report of a sub-committee of the Council, drawn up by Conrad 

 Beck, F.R.M.S., Secretary to the Sub-Committee. Read June 17th, 1896." 



"The so-called Standard Screw-Thread of the Royal Microscopical Society 

 has been but an imperfect standard, and has not ensured that interchangea- 

 bility which it originally promised. It has been our duty to investigate the 

 causes of this state of affairs, and to formulate a plan by which such an incon- 

 venience should be remedied in the future." 



" Without going too closely into the entire history of the subject, we pro- 

 pose to briefly explain the reasons why the original standard was not efficient 

 for practical purposes, and then to state the plan which the Council of the 

 Royal Microscopical Society has now adopted for the future." 



The specification of the original standard screw was as follows : 



§ 136. Form of Thread. — " Whitworth thread, i. e., a V-shaped thread, 

 sides of thread inclined at an angle of 55° to each other, one-sixth of the V 

 depth of the thread being rounded off at the top of the thread, and one-sixth 

 of the thread being rounded off at the bottom of the thread." 



" Pitch of Screw, 36 to the inch. 



Length of Thread on Object-Glass, 0.125 in. 



Plain Fitting above Thread of Object-Glass 0.15 in. long, to be about the 

 size of the bottom of male thread. 



Length of Thread of Nose-Piece not less than 0.125 in. 



Diameter of the Object-Glass Screw at the bottom of the screw, 0.7626 in. 



Diameter of the Nose-Piece Screw at the bottom of the thread, 0.8 in." 



"When the exact form of the Whitworth screw-thread is calculated it will 

 be found that this allows a difference between the male and female screw of 

 0.0018 in., which is in itself quite sufficient margin of looseness to make an 

 easy fit." 



" The society had two plug and ring gauges, one 0.8 in., and the other 

 o 7626 in., made by Whitworth as standards for the use of the Society, and it 

 has been shown that if an adjustable tap and die (as recommended by the late 

 Mr. Richard Beck in a paper printed in the "Transactions of the Microscopi- 

 cal Society," 1859, p. 92) be made which could be accurately adjusted to these 

 standard sizes so that the tap exactly fitted the 0.8 in. ring size, and the die 



