A few flakes of an opal glaze which showed pinkish veins when examined visually, could only be seen as of a brownish-yellow colour with ordinary transmitted light, and as bluish-white by ordinary dark-ground illumination or with the ring illuminator. Transmitted polarized light, even when the nicols were crossed, gave a good deal of general white light owing to the intensity of the opal, and the pinkish colour was completely masked. With the ring illuminator used to reflect polarized light on to the speci men (the polarizer being below the stage as usual), large numbers of coloured particles could be seen when the nicols were crossed, and these particles were of the unmistakable colour of copper.
With the nicols parallel, a few of these were distinguishable owing to their having been previously located when the nicols were crossed, but the colours were so much paler that it would have been impossible to identify the nature of the particles by their colours. Under similar conditions crystals of ferric oxide in a glaze were easily recognized by their deeper red or brownish-red colour.
In a green-gray porcelain "body" of presumably Chinese origin, a few pale-green particles could be detected by careful observa tion of the porcelain, using ordinary light and a ring illuminator. With polarized light and the ring illuminator a very large num ber of such particles could be seen, amply sufficient to account for the depth of colour seen when the porcelain was examined visually. Moreover, the colour of the particles was so clearly recognizable that it gave strong presumptive evidence that the particles were a compound of chromium. Subsequent spectroscopic examination identified chromium.
Examination of a tarnished piece of copper by the vertical illuminator showed a flush of colours such as are produced by thin films. With the ring illuminator the whole surface appeared granu lar except for a few scratches, most of the granules appearing like burnished copper, though a few were blackish. The interference colours were not seen. Examination under the ring illuminator using polarized light, and with the nicols crossed, showed that most of the granules were more crimson in colour than the surface of copper seen in the scratches, and these granules were recog nizable as particles of cuprous oxide. The blackish particles showed an intensified darkness of colour; these were particles of cupric oxide.
Diatoms mounted in Canada balsam or styrax are hardly visi ble when viewed by transmitted light between crossed nicols, but if mounted dry or in realgar they show up brightly unless the main lines in the structure lie parallel to the axis of one or other of the polarizing prisms. As the diatom is rotated, extinction
positions highly reminiscent of those obtained with doubly refract ing crystals occur, extinction, in the sense of greatly diminished brightness, occurring every 90° of rotation. Reflection surfaces give similar extinctions, and it is very easy to mistake a reflecting surface of any material, whether doubly-refracting or not, for a fragment of a doubly-refracting crystal.
This method of illumination is of very great value in critical microscopy, as can be gathered from the few examples which have been quoted. It is also useful when an object is to be examined with a cone of transmitted light large enough to fill the object glass aperture, as it enables this to be done without incurring such loss of contrast as results from light reflected by the surface of the cover-glass (cover-glass glare) or by the surfaces of the indi vidual lenses in the object-glass (lens flare). This reflected light is almost entirely stopped by the analyser and, as a result, the full resolving power of the object-glass can be obtained by full use of the object-glass aperture, without any marked loss of con trast such as is usually observed with ordinary transmitted light and a similarly wide-angled cone of illumination. The general appearance of the image when the object is examined between crossed nicols with transmitted light resembles that obtained with dark-ground illumination.