The plate, or film, is now put in the _fixing bath_, which is simply a solution of _hyposulphite of soda_ and water. This fixing bath prevents any further action of the light on the plate, or film. But the picture on the glass plate, or celluloid film, looks all same like a Chinese chromo, for the light and dark parts are just the reverse of that of the object which was photographed; in other words, where the picture should be white, it is black, and where it should be black, it is white, and this is the reason it is called a _negative_. [Illustration: FIG. 180.--CAMERA POINTING TO NORTH STAR.] To get a picture of the object, as it looks to the eye, a sheet of paper, also coated with salts of silver, is laid flat on the negative and held close to it in a _printing frame_ when it is exposed to the light of the Sun. When the paper is printed dark enough by the Sun, it is _toned_ to give it a pleasing color and _fixed_ to make it last a long time, and when washed and dried the image of the object, true to life, is there, a wonderful record for all time, nearly. This then, briefly, is how the Sun, or other source of light, makes an image, how the image is fixed on a dry plate, or film, and how a picture is printed from the negative, and having found out this much let’s see how we can make it do a little star work for us. There is not much we can do in the way of making pictures of the things in the sky with a small camera, but the few things we can do are mighty interesting, and show the stars in a way you can never see them with the naked eye. To make _star trails_ is one of these interesting things. First, set your camera on a _tripod_, or other firm support, and focus it on a tree or something as far off as you can see it in the daytime. Now, on a dark night, when there is no Moon, set your camera so that the lens is pointing directly at the North Star, as shown in Fig. 180. Set the shutter of your camera for a _time exposure_ and open it. [Illustration: FIG. 181. STAR TRAILS.] You can go to bed now and let the stars work for you while you sleep, that is if you can get up before daylight the next morning, but if you cannot do this stay up as long as you can, three or four hours, anyway, before you close the shutter. Now, when you develop the plate, or film, and make a print from it you will have a record of the apparent path of the stars, as shown in Fig. 181, but which is, of course, due to the Earth turning round on its axis. If you set your camera with the lens pointing toward the ecliptic, that is, the path of the Sun, the star trails will be long, straight lines, and this will make another interesting record.