7 8 LIGHTING AND FOCUSING \_CH. II 



exactly fitted the 0.7626 in. plug, the exact standard screw as originally sug- 

 gested could be adhered to. These adjustable taps and dies were not used for 

 cutting the thread, but for passing over each thread after it had been cut to 

 approximately the right size. That this method will work satisfactorily, is 

 evidenced by the fact that in the late Mr. Richard Beck's firm the method has 

 been in successful operation ever since." 



"The use, however, of such a system involved the necessity of every maker 

 being provided with adjustable tap and die, and also the two pairs of plug and 

 ring Whitworth sizes, together with a means of accurately sharpening the 

 adjustable tap and die. And it was found in practice that microscope makers 

 were not universally prepared to go to such an outlay for a .matter which at 

 that time did not appear to be of such importance as has since proved to be 

 the case." 



"Therefore the Society issued solid taps, and finding that, as is well known 

 to be the case, a solid tap could not be made to an exactly accurate size "owing 

 to the alteration of the steel during the process of hardening and tempering, 

 they had them made somewhat larger than the standard 0.8 in. gauge. An 

 additional reason for their being larger was to allow for the slight wearing of 

 the tap after prolonged use." 



" Here, however, there was no record of the amount larger which the taps 

 were made, and although the first set appear to have been carefully manufac- 

 tured, those which were from time to time obtained were less and less like the 

 original, and in this manner a discrepancy arose which the arrangements now 

 adopted by the Council are intended to correct for the future." 



" Beyond the fact that the Council specify that the diameter of the plain 

 fitting of the object-glass should be as near as possible to, but not exceeding 

 0.759 * n -> an d that the length of this fitting has been reduced to 0.1 in., the 

 original specification of the standard screw is only altered as to the exact 

 diameters of the screw itself." 



" The original specification of these diameters allowed only 0.0018 for 

 clearance between the male and female screw." 



" If absolutely exact sizing taps and dies could be made which should not 

 wear, the original diameters might have been adhered to, but as has been pre- 

 viously pointed out, adjustable dies in connection with gauges, etc., are 

 requisite for this." 



" The Council has been able to obtain taps and dies which are guaranteed 

 not to vary more than 1/1000 of an inch larger or smaller than the nominal 

 size. And they are therefore having manufactured a series of taps of the 

 nominal diameter on the top of the screw-thread of 0.8015 i n - which will not 

 vary more than from 0.8005 i n - to 0.8025 in. To insure this the Council has 

 ordered a Whitworth plug and ring, size 0.803 i n - "> diameter, and no tap will 

 be allowed to be stamped with the Society's stamp unless it will pass easily 

 through this 0.803 ' n ' ring, and unless it is of such a size that it will not enter 

 the 0.8 in. standard gauge already in the Society's possession." 



"They are also having made a series of dies of the nominal inside diameter 

 on the top of the thread of 0.7611 in., which will not vary more than from 

 0.7601 to 0.7621. To test this the Council has ordered a Whitworth plug and 



