40 
Report of Bub -Committee. 
3. That the fitting must be capable of construction in an 
ordinary lathe. 
In order to ensure parallelism of the axes_, a face-fitting is 
generally considered necessary. It also appears desirable 
that the inside fitting should be in the body of the micro- 
scope^ and the outside fitting on the object-glass. Of the 
various modes of attachment that have been suggested^ that 
which appears likely to fulfil most completely the conditions 
of perfect centerings is a cone of about 40°_, surmounted by 
a screw which enters a loose nut placed above the hollow cone 
in the body of the microscope^ but the practical difficulties 
of manufacture appear insurmountable ; it is therefore pro- 
posed to relinquish the greater degree of accuracy that might 
thus be obtained^ in favour of a mode of fitting that is at 
present partially in use^ namely^ a screw^ surmounted by a 
plain collar or guide, for facilitating the application of the 
object-glass. As the correct centering must practically de- 
pend on the screw, it is strongly recommended that the inside 
and outside screws should both be cut by a traversing man- 
drel, or by a traversing slide-rest. 
Having thus considered the form of the attachment, it re- 
inains to determine the most appropriate dimensions of the 
several parts. A screw, containing thirty-six threads in an 
inch, having an angular thread of 54°, slightly rounded oflP 
at the top and bottom, has been considered the most appro- 
priate. The largest linear aperture, at the junction of the 
object-glass with the body of the microscope, will be required 
for objectives of low power having the widest compatible 
angle of aperture; this is not likely to exceed '72 to '73 in. 
with the greatest diameter of field-glasses now in use ; hence, 
•8 in. may be taken as sufiicient for the external diameter of 
the screw. The length of screw recommended is \, or "125 
in., comprising 41^ threads ; and that of the guide or collar 
•15 in. 
In order to ensure uniformity of dimensions among difife- 
rent manufacturers, it has been thought desirable that an 
application be made to Mr. Whitworth to construct the 
requisite number of hardened gauges, of exactly equal dimen- 
sions, and he has kindly undertaken their construction. 
The proposed set of gauges consists of a templet and ring 
of exactly '8 in. external and internal diameters respectively; 
another templet and ring corresponding in diameter to the 
bottom of the thread, which Mr. Whitworth has determined 
to be '7626 in., to be used as gauges for the plain parts of 
the fitting; and a master-tap or ^'^hob" for cutting screw 
tools. 
