With this simple appliance it is easy to study the details of the thinnest clouds right up to the sun, and even the image of the sun itself may be glanced at without serious discomfort. Nor is the general diminution of brightness the only gain. If the glass is so held that the light from the cloud makes an angle of about 33 degrees with the surface, some of the blue light from the sky is suppressed altogether, while that from the cloud is practically unaffected. The exact fraction suppressed depends upon the part of the sky relative to the sun, and also on the position of the mirror, but a few minutes’ trial will show when the maximum effect has been reached. It is astonishing to see for the first time how the delicate filaments of cirrus or the beautiful structures of cirro-cumulus stand out shining white on the deep blue background; and the use of the black mirror is a revelation to most. It also has one indirect advantage, which is really more important than it seems. By gazing down into a mirror long-continued observations can be made, and one form of cloud may be watched changing into another, and possibly back again into its original shape, without any danger of incurring that unpleasant result of much looking upwards which is sometimes known as exhibition headache. Such a mirror may be quite small, so that it can be carried in a pocket-book, a point of some moment, as many of the forms of cirrus are exceedingly transient, coming and going in a few minutes, while others are in a state of continuous change. This is particularly often the case with the exquisite ripple clouds, and the delicate lacework of the higher kinds of cirrus. Still another advantage possessed by the mirror is that it makes it easy to see the solar halos formed on the verge of a cyclone, and to detect their iridescent colouring in a way which is quite beyond the reach of the naked eye or any protective spectacles. Every one is familiar with the faint halos formed round the moon, but the corresponding solar phenomenon is comparatively little known, though it is far commoner, much more brilliant, and often glows with colour. Its very brightness, and that of the background on which it is projected, hides it from the eye, except on those rare occasions when the sun is conveniently hidden by some thicker cloud. If some permanent record is desired, much can be done with a few light strokes of a pencil, but more ambitious pictures are best secured by the use of soft pastels, aided by a liberal use of the finger or leather stump. Ordinary paints, whether oil or water-colour, are of little use for actual study of cloud detail, except in the hands of a highly skilled artist who knows how to get the effect he wants in the minimum of time.