Non-homocentric Pencils. 715 
actually crossed the pinhole. The plano-convex lens was 
placed 43 cms. beyond this with its convex side towards the 
incident light. A vertical wire was placed in the emergent 
beam about 18 cms. from the lens, and its shadow received on 
a photographic plate 15 ems. beyond. As the shadows were 
heavily fringed with colour, a coloured screen was placed 
before the lamp. This consisted of a parallel-sided glass cell 
5 mins. thick, containing equal volumes of a 0°5 °/, solution 
of acid green in water and 0:1 °/, solution of tartrazin. This 
was recommended by Mr. A. J. Bull as transmitting only a 
short region in the blue-green of the spectrum, from about 
44900 to 25100, and proved very satisfactory. The plates 
were Westerndorp and Wehner’s Isochromatic. Fig. 9 
(Pl. XX VII.) shows a shadow obtained by this method with 
the wire not completely crossing the pencil, and illustrates 
the collinearity of the shadows of a point. 
There is another and a more convenient way in which 
these forms can be observed. If the eye takes the place of 
the illuminated pinhole, the disposition of the lens or mirror 
and the object-wire remaining the same, the form of the image 
of the wire that is seen is that of a section of the surface ruled 
by rays passing through the pupil and the object-wire, which 
is that of the virtual shadow on the surface of the lens or 
mirror, and practically identical with that of the real shadow 
on a plane beyond the wire. Thus the forms can be seen 
with great clearness by looking at the reflexion in a concave 
mirror of a straight wire held suitably between the mirror 
aud the eye. On moving the eye from side to side the image 
passes through the same phases, as shown in fig. 4, while if 
the eye be moved nearer to or farther from the mirror, the 
effect of moving the wire in a direction parallel to the axis is 
imitated. If two eyes are used two of the phases are seen 
simultaneously. Similar results can be obtained by looking 
at a wire through a suitable lens ; the chromatic effects are 
more in evidence if the wire is replaced by the straight 
filament of an incandescent lamp, or by an illuminated slit, 
so that the shadow-forms are seen bright against a dark back- 
ground. If the slit is replaced by a number of parallel slits, 
or by a rectangular grating, very complicated forms are seen, 
which are formed by the superposition of the curves due to - 
the various straight slits. I have seen the same effect by 
looking through the globe of a sunshine recorder at the 
rulings on the record strip beneath. 
This arrangement may also be used for photographing the 
shadows, the eye being replaced by a camera. The place of 
the pupil must be taken by the diaphragm of the lens, which 
must be stopped down to a small aperture. 
