The same question of “depth of focus” occurs in photography and perhaps an example showing how it affects the camera user may make the matter clearer. Suppose we wish to photograph a landscape having, let us say, a tree in the foreground, a cottage in the mid-distance and a hill in the distance. If our lens is one of large aperture, that is to say admits a considerable amount of light and is also what is known as a long focus lens we shall find, when we view the scene on the ground glass, that when the tree is sharply focussed, the cottage and hill are not clear. When we rack in the camera to get the cottage sharply focussed, the tree and hill will be un-sharp. Similarly when we focus on the hill the tree and cottage remain out of focus. The reason is that the lens in our case possesses little “depth of focus.” The experienced landscape photographer, did he wish all three objects to appear equally sharp could easily attain his object. He would perform the simple operation known as stopping down his lens, that is to say he would gradually close its diaphragm, while viewing the scene on the ground glass. At a certain point everything would be sharp, from foreground to distance. At the smaller aperture of the lens, caused by closing the diaphragm, the depth of focus would be considerably increased, at the same time much less light would be admitted to the camera. Looking upon our object, under the microscope, as comparable to a landscape, seen on the ground glass focussing screen of a camera, it is obvious that, unless our object has no thickness, and this is impossible, we cannot highly magnify its upper and lower surfaces at one and the same time. There are no adjustable diaphragms in the objective so our only course is to examine the two surfaces in turn or to resort to a lower magnification. [Illustration: _By the courtesy of Messrs. F. Davidson & Co._ THE MICRO-TELESCOPE 1. View taken with ordinary camera. The arrow shows the building illustrated in the plate below; it is 3/4 mile away. 2. The building indicated by the arrow in the plate above, taken from the same standpoint through the micro-telescope. ] Apart from any other consideration, the super microscope marks a big advance from the fact that it possesses great “depth of focus.” It is possible, for example, with this remarkable instrument to examine a moss as it grows, with a high magnification and see not a portion of a leaf or a fragment of the stalk, as with the ordinary microscope, but the whole upstanding plant, in stereoscopic relief. It shows us objects exactly as we should see them were we endowed with super eyes, enormously enlarged, in relief and erect. Objects viewed through this instrument are not inverted, as with the ordinary microscope. [Illustration]