AucusT 7. 1902] 
can be made to serve, but it is useful to have a sliding tube in a 
brass upright and at the topa V whichis above a stiff joint. On 
this V the eye-piece can be held with an elastic band ; or an eye- 
piece of different breadth, ora Nicol’s prism may be so fixed. 
Most of the effects may be well seen if a lamp is used for the 
source of light. It is easier to observe in a room which is rather 
dark, for then the space between screen and eye-piece need not be 
Fic. 4. 
covered up. Sunlight may be reflected by a hand-mirror into a 
darkroom through an open door. The lens which condenses on 
to the pinhole can be moved a little until it is seen that a pencil 
of light is just covering the Jens of the eye-piece ; then the object 
can be adjusted so as to be also covered by this pencil. Of course, 
the effects as seen directly by the eye are far more beautiful than 
the photographs. The object is about 18 inches from the pinhole. 
With good sunlight there are often groups of brilliant jewels, in 
which the emeralds and the rubies are made to change places by 
a small movement of the object. For producing Arago’s bright 
spot with an object so broad as a threepenny piece, the distance 
from the pinhole to the eyepiece must be about 36 feet. 
In photographing the phenomena the same eye-piece is fitted 
on to the front of a camera, instead of the usuallens ; the camera 
can be placed ona box and adjusted to the right height with 
books or paper. If there is good sunlight, about 30 seconds’ 
exposure is suitable for an Ilford ordinary plate. There is no 
question of focus ; to whatever position the back of the camera is 
drawn out the figures are equally in focus. There is a series of folds 
or zones of light and shade, and prismatic colours which are 
ofien crossing one another and suffering interaction of their 
waves, so that if these zones are cut at various positions there is 
infinite change of form and colour. If the same eye-piece is used 
in a telescope or a microscope it will focus the sun or a diatom 
on to a screen ; and when the screen is moved back the image is 
still in focus; but in these cases the reason is different, for the 
pencils of light emerge in parallel rays and make clear images 
at all positions. 
Some of us who are fortunate have been inspired in early life 
at some well-known schoul of optics where there was the best 
Fic. s. 
teaching and the best instruments ; then we descend to our own 
rough contrivances.. There is a third stage, when we watch for 
examples of wave interference with no apparatus at all. In 
looking at distant lamps at night-time several forms of Grimaldi’s 
fringes can be seen by moving the eyelids over the eye; with 
care it is possible to see interference from the light reflected 
along the eyelashes, and there are fine figures of endless variety 
xO. 1710, VOL. 66] 
NATURE 
355 
which can be produced by slowly lifting the eyelid so as to 
draw a film of moisture across the eye. Some are similar to 
the figures of Michelson’s refractometer, and all these are 
probably of the character of thick plates. 
Polartsation.—It has often been considered that the rings and 
brushes made by oblique pencils through crystals can only be 
seen with elaborate instruments ; but according to the method 
here described all polarisation effects of which the writer has 
any cognisance have been photographed. It is convenient to 
cut the cork mounts of the crystals so as to be round, about 
7 inch diameter. Place one of these on the top of the same 
microscope eye-piece, from which the cap has been removed ; if 
the microscope is at hand, it forms a convenient holder. No 
objective is put on. A Nicol’s prism may be inserted at the 
bottom of the tube. This, however, is not necessary, for the 
reflection from the plane mirror under the stage polarises almost 
as well. A cardboard tube, which fits the eye-piece, may 
be used instead of the microscope. In this case, cut a hole in 
the side of the tube at the further end and reflect up polarised 
light by a piece of microscope cover glass fixed on cork with 
black sealing-wax. A long pin run through the tube and the 
cork makes easy adjustment for this reflector. 
For an analyser a tourmaline is desirable, as light in colour as 
possible. The artificial tourmalines are best, but they are now 
difficult to procure. The tourmaline is placed above the crystal 
section which is already on the top of the eye-piece. Perhaps 
the cap of the eye-piece will hold these two steady when they 
are turned to the right position. It is safer to twist thin copper 
wire round the top of the tube so as to have a small hook on 
either side; a thin india-rubber band from hook to hook over 
Fic. 6. 
the tourmaline holds them securely. It is only necessary 
to place the tube through the front of a camera from which the 
lens is removed so as to have the figures projected on the photo- 
graphic plate. Sunlight is best to work with. Sodium light 
gives a field covered with fine detail, but does not make effective 
lantern slides. 
Fig. 4 shows the calcite cross ; Fig. 5 shows calcite circularly 
polarised and circularly analysed, or circularly polarised and 
plane analysed. One of the most attractive of polarisation 
crystals is Bertrand’s prism. It is so cut that light entering 
directly through one side is internally reflected along the optic 
axis ; it then emerges owing to a similar internal reflection. 
These reflections polarise and analyse ; moreover, the first 
reflection preserves both the ordinary and extraordinary rays. 
The result is that by placing this one crystal without polariser 
and analyser on the eye-piece the black-cross and the white- 
cross systems both appear at once, as seen in Fig. 6. The 
interest does not end here; with a slight tilt, made by a 
shaving of cork under one side of the prism, the white and 
black crosses change places, with certain other changes of 
detail in the figures ; and there is another tilt which will give 
two white-cross systems. 
If Fig. 6 is to be projected on to a screen ona large scale, 
the crystal is placed at the focus of a convex lens on to which 
sunlight is turned, or else at the focus made by the condensers 
of a lantern when the light is drawn back ; this is sufficient 
without a focussing lens. In both cases the heat may injure 
the crystal unless about 4 inch of water is placed between 
the lens and the crystal. The ordinary crystals of calcite and 
nitre must have a Nicol on one side and a tourmaline on the 
other, and be placed at the focus as before. W. B. CRorv. 
