208 Dr Searle, Experiments illustrating 
too faint to produce a noticeable effect during the short exposure 
employed with a large stop, it may be quite strong enough to. 
produce a deleterious patch upon the plate during the long 
exposure required to obtain an image of the background with a 
small stop. There are many lenses which cannot be used with a 
small stop for this reason. A flare spot may be detected by 
watching the focussing screen as the opening of the stop is 
diminished, the camera being pointed towards the sky or other 
bright background. 
If the edges of the stop are bright, they may reflect light 
forwards and this light after being reflected at one of the surfaces in 
front of the stop, may come more or less nearly to a focus on the | 
plate. Since this light has suffered only one reflexion at an air- | 
glass surface, it may produce a strong image or at least a consider- | 
able fogging of the plate. The same remark applies to the tops 
of screw threads, which may have been worn bright, and to the 
edges of the lenses themselves. The defect may be cured by 
properly blacking the surfaces with good optical black. . 
| 
: 
§ 4. Haperiment with spectacle lens. A virtual flare spot 
is easily seen when a positive (convex) spectacle lens of focal — 
length f is placed before the eye. Ifa bright object such as a gas 
flame or the bright sky is viewed through a small hole cut in a | 
sheet of paper, a well defined secondary image of the hole will be | 
seen when the distance between the lens and the hole is about 
jf /7 (see § 7). The secondary image of the hole is virtual and at a 
considerable distance from the eye. By adjusting the position of - 
the hole the secondary image may be seen clearly at the same 
time as the flame. This corresponds to the case where a secondary 
image of the stop of a camera is focussed on the plate at the same 
time as the primary image of distant objects. 
If two pairs of spectacles are worn, six secondary images can 
be seen. 
§5. Thin lenses in contact. The positions and magnitudes 
of the secondary images of any object formed by a system of any 
number of thick or thin lenses can be calculated by repeated 
applications of the formulae for reflexion and refraction at spherical 
surfaces. When, however, the system consists of a single thin lens — 
or of two thin lenses in contact, the positions of the secondary 
images can be very readily calculated by the method of least time. — 
The experimental tests of the formulae can be made with simple — 
apparatus and form interesting exercises for students who have 
made a little progress in practical optics. 
§ 6. Focal length of thin lens. The application of the principle 
of least time to find the focal length of a thin lens is well known, 
