The camera swings on a couple of screws, which act as trunnions. These pass through two upright arms, which spring on either side from the base board, which is attached to the ordinary camera stand. This base board can be rotated into any horizontal position desired, and the camera can be tilted through any vertical angle by swinging it between the uprights, and can be clamped by tightening the two trunnion screws. These screws are so placed on the front of the camera that the lens and its attachments on the one side nearly balance the back part of the camera on the other side, and so lessen the danger of slipping. Supported in front of the lens by light brass-work is the black mirror, made of a very dark glass optically worked on the front face. It is a curious fact that, although bits of plate-glass blackened on the back seem to the naked eye to give a single image of sufficient truth, if such a mirror is placed in front of the camera the second faint image formed by reflection from the blackened surface is almost always to be detected. Moreover, the lens with its large aperture at once detects irregularities in the surface of the glass, which are quite imperceptible through the narrow limits of the pupil. Black glass, with a truly worked surface, is essential then, but the surface need not be of the high order of excellence required for mirrors used for telescopic work, since the first image is not, as a rule, intended to be highly magnified. The mirror is held so that its surface makes an angle of about 33 degrees with the axis of the lens, and the block carrying shutter and mirror can be turned round into any position by slipping it round the lens mount as an axis. The mirror thus always retains the correct angle. The action of the mirror is to a large extent due to mere diminution of brightness, but it also partly extinguishes the blue light of the sky without exerting any such influence on the white light from a cloud. This is due to the fact that the blue light of the sky is partly polarized, while that reflected from the cloud is not. Now, polarized light which falls upon a black mirror held in a particular position is not reflected by it. This position depends upon various circumstances, but one condition is that the reflected ray must make an angle of about 33 degrees with the surface of the glass. The amount of the polarized component of the blue light varies greatly, but is at a maximum at all points 90 degrees away from the sun. This, then, is the best possible position for photographing a cloud, as the whole of this polarized component may be suppressed by adjusting the mirror to the proper position, and then the most delicate cirrus fibres stand out brilliant on an almost black background.