Of these, of course the slow plate and small aperture are the simplest to adopt, and all the cloud studies shown in the illustrations to these pages have been taken on plates prepared for photo-mechanical purposes or for transparencies. There seems to be nothing to choose between these two brands. Orthochromatic, isochromatic, double-coated, and many other special types of plate had previously been tried, both with coloured filters in front of the lens and without them, without showing any marked superiority over an ordinary plate of low rapidity. At last the photo-mechanical plates were tried, and the efforts made to get satisfactory cloud portraits, which had previously been marked only now and then with satisfactory results, became uniformly and continuously successful. If the slow plates are exposed in the camera without either a screen or the black mirror, the diaphragm should be reduced to a small size and the exposure suitably adjusted. The length of exposure may generally be judged by looking at the image on the focusing screen, and reducing the aperture until the picture shows its detail easily. Then, regarding the picture as that of a sunlit sea or distant landscape, judge the necessary exposure by the brightness of the image. No definite rule can be given. The light varies enormously from day to day, and hour to hour, and especially with the position occupied by the cloud relative to the sun. Thus, working with a lens of six inches focus and an aperture of a quarter of an inch, the exposure may vary from the quickest snap of a Thornton-Pickard roller blind to as much as a quarter of a second, or even more. Again, using a lens of eighteen inches focus and an exposure of a fiftieth of a second, the necessary aperture might vary from an eighth of an inch up to an inch and a half. But if we suppose that we are dealing with an ordinary bright summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus or cirro-cumulus, an aperture of about one thirty-second of the focal length will probably give some sort of image with a snap-shot exposure. At first the failures will be many, but a little practice will soon enable very respectable pictures to be taken by varying either the diaphragm or the speed of shutter. Heavier clouds of the alto types will need rather longer exposure or larger aperture.