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sequence the drum division equivalent as well. The readers used for the experi- 
ments described later in this paper, when fitted with x4 objectives, have drum 
equivalents of 0. 955 micron per division. This equivalent corresponds to tube . 
factors greater than 1. 
Figure 1 shows the graticule of the measur- 
ing eyepiece. The scale, a, has 12 divisions, 
each division being equal to 100 divisions (or 
one revolution) of the micrometer drum. À 
fixed line, b, passes through the fourth grati- 
cule scale graduation. c is a movabie lateral 
line controlled by the reader drum and, d, is 
a short longitudinal line marking the middle 
of ¢ and which moves with c; the c-d intersec- 
tion provides the measuring mark. The grat- 
icules of both readers are identical and the . 
widths of all graticule lines are slightly less 
than 2 microns, with x4 objectives. The eye- 
pieces may be rotated in their respective 
tubes to set the c lines parallel to the scale 
| graduations. Two heavy-based stands to 
Figure 1 - Micrometer field carry the readers and a source of transmitted 
of view with main scale and light, i.e. a light box, completes the appara- 
pre-marked point in position tus. ! 
for measurement 
70 Se 4o /oo "Oo a 
  
  
Measurement of the diapositives is carried out monocularly. The glass plate, 
or film, emulsion up, is placed inside a transparent plastic envelope. This 
envelope fulfils a dual purpose, (i) it protects the emulsion from scratching by 
the scale, and, (ii) it provides sufficient friction between the glass top of the 
light box and the diapositive, and also the scale and the diapositive, to prevent 
slip between these and permit accurate measurement. The glass scale is 
placed graduations down on top of the plastic covered plate. Observation is thus 
carried out through the scale. Correét alignment of the scale is necessary. In 
Table 1, the effects of some alignment errors over various distances are shown. 
It is usually possible to keep scale alignment correct to 0.1 mm, but as will be 
seen from the Table greater care is required when measuring the shorter dis- 
tance. Distances shorter than 40 mm usually occur only once or twice for each 
plate. 
The greatest objective magnification which can be used is limited by the focusing 
distance of the microscope. With a 325 mm long tube a x4 objective is the highest 
power that can be used with the x15 reader. The distance between the objective 
and emulsion is less than 5 mm, i.e. less than the width of the scale, with a x6 
objective. It would therefore be impossible to focus the microscope with a x6 
objective with the scale placed on top of the glass plate, or film. Depth of focus 
of the readers, about 200 microns with a x4 objective, controls the thickness of 
the plastic which may be placed between the scale and the emulsion. During 
measurement the photo-point and scale graduations must both be in clear focus 
and there must be no parallax between the respective images. 
Positioning the scale and plate under the readers and measuring the distance be- 
tween two photo-images takes about 20 seconds. Fractional parts of the main