aperture of the stop of a photographic lens Lt 
Hence, a ray RQ,, which before incidence on the lens LZ is parallel 
to the axis AF, and is directed to a point Q, on the edge of 7;, 
will, after traversing LD, pass through P, the corresponding point 
on the edge of the stop itself. Hence it is clear that the distance 
of the ray RQ, from the axis AF, is equal to the radius (4a) 
of the effective aperture. 
Tf the front lens LZ is a converging lens, and if the stop S les 
between ZL and the focal plane of L, the image 7, will be further 
from Z than S is and consequently the diameter of the aperture in 
the image will be greater than in the actual stop. 
In Fig. 2, T, is the image of the stop S formed by the lens J. 
Each of the rays RB and UC in its path through the system passes 
actually, or at least in direction, through (1) Q,,(2) P, (8) the edge 
Q. of T;. It will be seen that &, is the image of Q, by the complete 
lens system. 
§ 4. First method. The effective diameter of the stop is easily 
measured by aid of a microscope mounted on a sliding carriage, 
such as that shown in Fig. 3. The lens system is firmly supported 
with its axis horizontal. The microscope 1s attached to the carriage 
so that its axis is at right angles to the direction of motion of the 
carriage, and the track on which the carriage slides is placed so 
Fig. 3. 
that the axis of the microscope is parallel to that of the lens 
system, the two axes being at the same height above the table. 
The microscope is then focussed through: the lens Z upon one end 
of the horizontal diameter of the stop, and the sliding carriage 1s 
adjusted so that the image of the edge of the stop is brought to 
coincidence with the cross-wire of the microscope or with a selected 
dividing line of the micrometer scale. Some adjustment of the 
microscope in the direction of its own axis will be required; the 
adjustment is complete when there is no parallax between the 
image and the cross-wire. The carriage is then moved along its 
track so as to bring the image of the other end of the diameter to 
the cross-wire or to the same micrometer division as before. The 
distance through which the microscope has been moved is equal 
to a, the effective diameter of the stop. After each pair of 
