_Directions for finding the Plane at which the Sharpest Image can be obtained._--Non-Achromatic Lenses are understood by all to require correction for the chemical focus; but it is usually said of the compound glasses, that their two foci correspond. The amateur is recommended, in order to avoid disappointment, to test the accuracy of this statement, and also to see that his Camera is constructed with care. To do this, proceed as follows:-- First ascertain that the prepared sensitive plate falls precisely in the plane occupied by the ground glass. Suspend a newspaper or a small engraving at the distance of about three feet from the Camera, and focus the letters occupying the centre of the field; then insert the slide, with a square of _ground glass_ substituted for the ordinary plate (the rough surface of the glass looking inwards), and observe if the letters are still distinct. In place of the ground glass, a transparent plate with a square of silver-paper which has been oiled or wetted, may be used, but the former is preferable. If the result of this trial seems to show that the Camera is good, proceed to test the correctness of the Lens.-- Take a Positive Photograph with the full aperture of the portrait Lens, the central letters of the newspaper being carefully focussed as before. Then examine at what part of the plate the greatest amount of distinctness of outline is to be found. It will sometimes happen, that whereas the exact centre was focussed visually, the letters on a spot midway between the centre and edge are the sharpest in the Photograph. In that case the chemical focus is longer than the other, and by a distance equivalent to, but in the opposite direction of, the space which the ground glass has to be moved, in order to define those particular letters sharply to the eye. When the chemical focus is the shorter of the two, the letters in the Photograph are indistinct at every portion of the plate; the experiment must therefore be repeated, the lens being shifted an eighth of an inch or less. Indeed it will be proper to take many Photographs at minute variations of focal distance before the capabilities of the lens will be fully shown. The object of finding the point at which the sharpest image is obtained will also be assisted by placing several small figures in different planes and focussing those in the centre. This being done, if the more distant figures come out distinctly in the Photograph, the chemical focus is _longer_ than the Visual, or _vice versâ_ when the nearest ones are most sharply defined. _The Single Achromatic Lens._--A useful lens for landscape Photography is one of about 3 inches diameter and 15 inches focal length, which may be expected to cover a field of 10 inches by 8. With the lens, stops are supplied of various diameters, the largest of which will be useful in dull weather; the smaller when the field is required to be rendered sharp to the very edge.