flare spots in photography. 213 
blackened internally, and the lens is attached to this tube*. A 
second carriage bears a plate pierced by a pinhole and furnished 
with cross-wires S. The hole is illuminated by a flame. The 
third carriage bears a pin 7’ which may conveniently be adjustable 
in a plane perpendicular to the length of the bench, and T is 
adjusted to coincide with the secondary image of S. In making 
the adjustment, a lens of about 15 cm. focal length, held in a clip, 
is of much assistance. 
Fig. 4. 
The secondary focal length is best found by the minimum 
distance method. The minimum distance between the cross-wires 
and the secondary image is 4/, + ¢, where t is the distance between 
the principal points corresponding to the secondary image. The 
lens being “thin,” ¢ is negligible and hence the minimum distance 
may be taken as 4/f,. 
The measurement is very quickly made if S is placed at about 
27/7 from the lens; the secondary image of the cross-wires will 
then be at a distance of about 2//7 from the lens on the other 
side. The minimum distance is then easily found. 
The distance p is found by making a well illuminated pin 
coincide with its own image when the face A of the lens is 
turned towards the pin. A black surface should be placed behind 
the lens. The distance q is found in a similar manner. 
810. Practical example. The following results were obtained 
in an experiment by J. L. Barritt. A double convex spectacle 
lens of a nominal power of one dioptre was used. 
Primary focal length (found by plane mirror) = f= 101-85 cm. 
Hence, power = F'= 1/f= 0009818 em.~'= 0-9818 dioptre. 
Minimum distance between object and secondary image = 4/, = 59°62 
em. Hence /,=14:905em. and F,=1/f,=0-06709 cm... Thus, by 
(2), S 7, 
yy 1 29-81 
Pig earn ae SCE 7 
15218. 
* Fig. 4 shows two lenses attached to the tube for the experiments described 
in § 12. 
14—3 
