PROCEEDINGS OF TILE SOCIETY. 
425 
bat ; stings of bee ; sections of cinnabar and fossil coal ; stings of 
queen wasp ; micrometer scale. 
The instrument was then rotated so as to bring the polariscopo into 
operation ; the value of this portion of the apparatus, i. e. polariscopes 
in general, was that it enabled an optical examination to be made of 
phenomena which could otherwise only be demonstrated by mathematics. 
The subject of the polarization of light being one which had not yet 
been exhausted, it was consequently very difficult to explain the subject 
in simple language. As a rule it might be taken that a ray of light 
vibrated in all directions, but when it was polarized it was made to 
vibrate only in one, which constituted plane polarization, or it might be 
circularly polarized, or elliptically polarized. It could of course be 
easily imagined that if the polarization was in a vertical direction, and 
a crystal which would not allow vertical vibrations to pass through was 
placed in the path of such a ray, the light would be stopped. The 
object of the first of his series of illustrations would be to show that the 
effects produced by certain crystals, and other substances which could 
polarize light, was due to their being more or less in a state of strain. 
Strains take place in a substance in three directions at right angles to 
one another ; if they were all equal they might be said to balance each 
other and no polarization effects were then produced ; if two were equal 
there would be polarization of one kind, and if all were unequal they 
would produce phenomena of another kind. The following were then 
exhibited upon the screen : — A piece of calc-spar, to show the effects of 
interference ; a piece of glass placed under strain by pressure from a 
screw ; a Prince Rupert’s drop ; a piece of chilled glass ; pieces of 
glass in a state of strain (chilled) ; thin pieces of mica crossed ; figures 
of a butterfly and of “ Baker turned sweep ” cut in selenite ; polarized 
ray passed through various films ; selenite picture of plum with leaves ; 
crystals of benzoic acid ; section of labradorite ; bi-quartz, the prism 
being set so as to obtain a uniform tint on both halves, the equivalent of 
a solution containing 10 per cent, of sugar 20 cm. long was placed in 
the path of the ray, causing a difference in tint to be at once observed ; 
piece of quartz cut convex. 
The rays were then condensed so as to become convergent instead of 
parallel in order to show better the internal structure and condition of 
a crystal, and the following were shown : — Apopholite, a uni-axial 
crystal which exhibited the black and white cross and no colour ; piece of 
calcite, the emerald; sugar, cut perpendicular to one axis, a bi-axial 
crystal exhibiting one arm of the cross ; tartarate of potash ; crystal 
showing the crosses ; sections of quartz crossed ; colour rings in crystals 
showing two axes; a crystal of selenite, naturally bi-axial, but when 
heated it became first uni-axial and then bi-axial at right angles ; “ star ” 
sapphire, showing the six stellate rays produced when the crystal was 
placed in the path of a small parallel beam, a phenomenon from which 
the name was derived. 
A large convex lens was then fitted upon the projecting arms of the 
lantern, and a powerful parallel beam of light being thrown on the 
screen various diffraction effects were produced by means of a grating 
ruled 2000 lines to the inch, which exhibited the secondary spectra in a 
remarkably clear way. Similar phenomena were also shown by means 
of two such gratings crossed, and also by a circular grating. These 
1893. 2 g 
