94 BecKj on Universal Screw, 
cause most opticians^ when cutting up their screw tools^ hold 
them by hand, whereas an exact counterpart of the hob can 
Pig. 4. 
only be obtained by the use of a screw- cutting lathe, or 
somewhat similar means, so that in the process the screw- 
tool may be held in one position, and may travel at the same 
speed as the thread upon the hob. 
But that which I wish to draw more especial attention to, 
is the mode of cutting screws to one and the same size by 
measuring the tops of the threads with plain cylindrical 
gauges. 
My first trials were made with particular care. I used a 
screw-cutting lathe, and the tops of the threads of the inside 
and outside screws (fig. 1) were made to fit exactly the re- 
spective gauges (fig. 2) ; the first dozen or so screwed plea- 
santly together, but as the point of the screw-tool began to 
wear, although very slightly, the screws no longer fitted, 
and yet the error was not detected by the gauges, because 
the variation was at the bottom and not at the top of the 
thread. 
After various experiments I could only thoroughly correct 
this error by giving a last finish to the screws with the two 
rig. 5. Fig. 6. 
adjustable screw- cutting gauges (figs. 5, 6). In either of 
