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aperture of the stop of a photographic lens 201 
glass scale is substituted for the plate D, its divided face being in 
contact with the upright. e 
Though this arrangement is more convenient for daily laboratory 
, work than an actual camera, students who possess cameras find the 
work more interesting if they investigate the stops on their own 
cameras. 
A projection lens so mounted is a very useful piece of apparatus. 
If a cross-wire is fitted to the metal plate D, the arrangement acts 
as a good collimator. The spherical aberration will be small if the 
lens system is mounted on the board so that the end which is 
intended to face the lantern slide, when the lens is used for projec- 
tion, faces the plate D. If the lens is mounted the wrong way 
round, its performance will be much less satisfactory. 
_§8. Practical example. The following results were obtained 
with an optical lantern projection lens system. 
First method. 
The readings of the sliding carriage on the track, when the microscope was 
focussed on the two sides of the stop in turn, were as follows : 
Right edge of image of stop, 20°97, 20-91, 21°00 cm. 
Left edge of image of stop, 17°78, 17°72, 17°82 cm. 
Differences, 3°19, 3:19, 318 cm. 
Hence (§ 4), mean value of effective diameter of stop=a=319 cm. 
Second method. 
Diameter of aperture in metal plate=d,=0°80 cm. 
The readings on the glass scale of the edges of the bright patch were as 
follows : 
Right edge of bright patch, 9°28, 9°30, 9°35 cm. 
Left edge of bright patch, 5°97, 6°00, 6°04 cm. 
Differences, 3°31, 3°30, 3°31 cm. 
Mean value of diameter of patch=c,;=3°31 cm. 
It will be seen that c, is considerably larger than a as found by the first 
method. 
For the distance BQ, of the image of the stop from the plane of the glass 
plate the readings were: 
Microscope focussed on B, 11°96, 11°92, 11°93 cm. 
Microscope focussed on Q;, 8°52, 8°53, 8°50 cm. 
Differences, 3°44, 3°39, 3°43 cm, 
Mean value of BQ, =x7=3'42 cm. 
The focal length of the system was found by the revolving table method. 
The two readings on the scale were 46:20 and 3°02 cm. 
Hence f= (46:20 — 3:02) = 21°59 cm. 
Thus, by (5), 
a EM Bee a 013318 ems 
Fi 21°59 
