The lens may be of any kind, as long as it gives a well-defined image, but there are many advantages in using one of the rectilinear type provided with an iris diaphragm. A rapid lens is not needed; indeed, it has been pointed out that slowness is a very great desideratum, and if the camera is provided with a rapid lens it must be ruthlessly stopped down. For general cloud purposes the best kind of lens is a wide-angle rectilinear, but many occasions will present themselves on which a lens of longer focus will be wanted in order to give more insight into the details of some specially delicate clouds. If the lenses are good, and the focusing is accurate, enlargements will go a long way towards revealing the minuter structures, but the results can never be quite so well defined as a direct photograph in a long camera. A shutter will be essential, and it should be one which opens in the middle, or which travels across the lens. The shutters which are ingeniously contrived to give more exposure to the lower part of the picture than to its upper part are useless for the purpose in view. It should have some latitude of exposure, from about one-sixtieth of a second up to a full second or more. Then as to the camera. Any light-tight camera will do, and, as the objects will all be at a great distance, it may very well be a fixed-focus one, or may be kept set up and fixed in focus for a distant object. If not, on setting it up it should be focused on the horizon or most distant object possible, and not on the cloud itself. As, however, the clouds present themselves at all heights above the horizon, even in the zenith, it becomes necessary to have some means of pointing the camera in such directions. To a certain extent the ordinary stand does allow of tilting, but a special support which will allow the camera to be fixed firmly in any position is of the greatest convenience. If the study is meant to be at all prolonged, the best plan is to make a suitable camera, once for all, which can be left in fixed focus, so as to be always ready, and which can be directed with equal ease to any part of the sky, from the horizon to the zenith. If it is intended to use a black mirror, then a special mount becomes almost essential. Many of the most delicate of the photographs reproduced here have been taken with a camera of peculiar pattern, the structure of which is shown in Plate 61. The lens is an ordinary rapid rectilinear, and the stop used was generally one-sixteenth of the focal length. The shutter is a light slip of aluminium, which can be drawn across from side to side at any desired pace. The body of the camera is mahogany, with a bellows part for getting correct focus, but when once this was obtained the back was clamped to the tail-board and a little varnish brushed over the clamping screws. [Illustration: Plate 61. CLOUD CAMERA FOR STUDIES.]