9^ 
Beck_, on Universal Screw. 
may be made by a screw-cutting lathe ; but, for so fine a 
thread, screw-tools may probably be equally well hand-made 
by a practised workman. As very few opticians either pos- 
sess traversing lathes, or have had much experience in 
making screw-tools, it was thought more desirable to have 
some screw- tools made from Whit worth's hob by an expe- 
rienced workman, and supply them at cost price to the ma- 
nufacturers. 
' The practical difficulties that occurred in the use of the 
cylindrical gauges may be thus explained. It is manifest 
that if the inside and outside screws were exactly the same 
size, that is, had exactly the same longitudinal section, they 
would fit each other as tightly as the cylindrical gauges, and 
would therefore be useless for the purpose proposed. One of 
three courses must therefore be adopted, — either 
1. The outside screw being made to the exact gauge size, 
the inside screw must be a little larger, — or 
2. The inside screw being made to the exact size, the out- 
side screw must be made a little smaller, — or 
3. Both inside and outside screws must be made to vary a 
little from the eocuct size. 
It soon occurred in practice, that object-glasses by one 
maker, who adopted the first course, would not enter the 
body of a microscope by another maker, who adopted the 
second course; and thus the proposed universality of the 
screw was so far set at naught. 
As both the top and bottom of the outside screw can be 
most easily gauged, it appeared to the committee more desir- 
able to adopt the first course, by giving a little ease to the 
inside screw ; and in order to ensure uniformity, to have a 
number of steel taps or gauges made of such a size, that if 
the body of the microscope were made to receive one of them 
tightly y an object-glass having an outside screw of the exact 
proposed dimensions would enter it easily and pleasantly. 
These taps must necessarily, for the reasons previously stated, 
be some two or three thousandths of an inch larger than the 
gauge size. 
It does not appear that the cutting points of the inge- 
niously contrived adjustible screw-cutting gauges proposed by 
the author of the paper possess any immunity from the 
same wear as that to which, as he justly remarks, the screw- 
tool is liable : nor would the mechanical efiect of setting out 
the cutting points difi"er from that of taking a slightly deeper 
cut with the screw- tool. 
The form of the inside fitting represented in the drawiiif 
that accoropaniee the paper, namely , with a plain -part -be- 
