NATURE 
[AveustT 7, 1902 
PHOTOGRAPHY OF DIFFRACTION AND 
POLARKISATIONVE EEE CLS, 
T was natural that such a subject as physical optics should 
call forth the hest skill of mechanicians, and science owes a 
debt to their beautiful instruments ; but the very excellence of 
these has perhaps filled the worker with too much awe and 
made him feel that wave interference can only be observed ina | 
great laboratory. The present object is to give details of simple 
arrangements which enable all such phenomena to be seen and 
photographed. 
Diffraction.—The general appearance of the apparatus is 
shown in Fig. 1. It may be seen at the Victoria and Albert 
Museum. The middle stand carries a square piece of soft 
wood blackened, 3 inches square, 4°; inch thick, with a } inch 
square cut out of the centre. A dozen or more of these wooden 
squares should be made, as holders for the various objects which 
cause diffraction. Itis desirable to have some sheet aluminium 
and rolled brass, the thickness in each case about No. 30 
standard wire gauge. Several objects can be made of aluminium 
and attached with pins to the centre of the wooden squares: a 
single edge; a rectangular edge for Grimaldi’s crested fringes ; 
two straight edges with adjustable distance, the adjusting edge 
of aluminium being made to slip under two clips of aluminium 
pinned down, one on either side. The pins should be cut short 
to about the thickness of the wood ; a slight tap with a hammer 
rivets them. Let three of the 3-inch squares have the central 
bic. 1. 
hole covered over with aluminium and let pinholes of various 
sizes be made; the aluminium is easily cut with scissors and 
pierced with a needle; when the hole is less than § mm. the 
phenomenon is different from that given by larger holes. 
Put thin microscope cover glass on the aperture of two or 
three wooden squares, and with the smallest speck of liquid glue 
attach shot of various sizes to the glass. This affords the 
easiest means of seeing Arago’s famous experiment, a bright 
centre in the shadow of an opaque circular disc. 
Other suitable objects are needles of various sizes, needle 
eyes and needle points. Fig. 2 shows diffraction bya fine and a 
thick needle. The centre of the shadow is a line of light in both 
cases ; it is the finer needle which has the broader central line. 
For the same cause the central light broadens towards the points, 
and the centre of the shadow ofa quartz fibre is very broad indeed. 
On one side of a needle place a strip of aluminium; this causes 
the interior bands to disappear at this position. To one side 
of a needle, along half its length, apply a piece of microscope 
cover glass; this shows Arago’s crucial experiment to prove 
that the velocity is less in glass than in air. It is imperfectly 
described by saying that the fringes are shifted towards the 
glass side. There is another system of fringes, narrower and 
more in number as if from a broader needle; the central line 
of these is shifted towards the glass. The experiment is rather 
difficult ; the edge of the glass gives trouble; it must be placed 
eae the thickness of the needle, and the needle had better 
e thin. 
NO. 1710, VOL. 66] 
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Perforated zinc and wire gauze easily give good effects ; the 
former has series of rings in the spaces, and the centre can be 
| made white or any colour with a small movement of the object ; 
Fic. 2. 
also there are fine hexagonal rulings all over the geometrical 
shadow. The wire gauze gives bright Scotch plaids of any 
clan, when moved over a range of about two inches. 
Some of the most beautiful effects which can be 
produced are shown in Fig. 3. The diffracting 
objects were small groups of four and five circles, 
circular lines of light photographed on glass, each 
group being 34; inch diameter. They were made 
years ago for another species of diffraction, that as 
studied by Fraunhofer, Schwerd and Herschel, in 
which a telescope is focussed on to a distant point 
of light and various screens are placed on the object 
glass. Our present view is that of Fresnel, in which 
waves starting from a point, almost mathematical, of 
light are diffracted or broken by an interposing body. 
A word more as to the holder of the objects. In 
the bottom of the 3-inch squares of wood, Fig. 1, is 
a slot; this slot is placed over a screw fixed in the 
top of the oblique bar in the middle stand ; a nut on 
the screw clamps the square and allows adjustment 
in a vertical plane. The lower end of the oblique 
bar, which is 3 inches long, has a stiff joint on to 
the upright, which is 9 inches high ; the stiff joint 
makes an easy adjustment for height. 
On the left of Fig. 1 isa wooden screen 16 inches 
high, 9 inches broad. At a height of 10 inches is a 
hole 4 inch in diameter ; on either side of this let 
there be a brass spring which will allow one of the 
3-inch squares to be slipped under it. The square 
should have a sliding strip of aluminium, with three pin-holes 
rangingin size from the smallest that can be made. Sometimes 
an adjustable slit should be placed here. A convex lens condenses 
lamplight or sunlight on to this aperture, whether pinhole or 
slit ; this is the source of light. 
Towards the right hand in Fig. 1 may be seen an eye-piece for 
| Fic. 3. 
direct observation or a camera for photographing the effects. 
The distance for either of these is about feet from the 
wooden screen which carries the source of light. The eye-piece 
is Beck’s B microscope Huyghen’s eye-piece. Any simple holder 
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