Beck Barnard ultra-violet microscope. 
Approx. } size. 
into play, and by rotating the milled head (v, Fig. 2), a motion 100 times as fine is obtained. The 
milled head carries a drum with 100 divisions so that one division on the drum represents a motion 
of the objective of -0001 m/m. (1/250,000inch). A drawtube (y) ground parallel to an exact cylinder 
and sliding in a ground fitting, is provided. 
The objective changer (z) is of a special type. It is an essential part of the method as it locates 
the objectives with such extreme accuracy that it is possible to focus the image seen by dark ground 
illumination with a first class apochromatic objective, and then to replace this with the quartz 
objective, alter the focussing adjustment by the required amount and obtain a sharp photograph 
with certainty and regularity. The essential feature of the changer 1s that the only force which retains 
the objective and its fitting in place is that of gravity. The objective is screwed into a circular 
collar (ar, Fig. 3) and is placed in contact with the back plate of (z). It is then allowed to slide 
downwards until the rounded corner of the collar comes into contact with the inside of the two 
steel cones (as). These hold the circular collar (av) against the back plate and also locate it so 
that the objective is accurately in the optic axis of the microscope. The two cones (as) are capable 
of rotation about an eccentric axis, and thus by rotating them, the objective can be moved 
laterally. In order to adjust a pair of objectives, accurately, so as to give the identical field of 
view, the rounded corner of the circular collar (a7) is rubbed down until the desired result is obtained. 
Provision is made to prevent the rotation of these collars when in position. The faces of the 
back plate and of the rings are optically flat and they are provided with recessed grooves into which 
dust is removed from their surfaces as the object glass carrier slides into position. An improved 
form has since been devised in which each object glass is carried in an adjustable frame in place 
of the rings above described. Three such frames are now supplied with each outfit. 
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