Procure a piece of stout card-board, or thin wood covered with white paper, on which draw a considerable number of fine black lines, or cover it with some fine black net (what I believe the ladies call _blond_), which may be pasted on. Cut up the whole into a dozen good-sized pieces of any convenient form, so that about four square inches of surface at least be allowed to each piece. Paste over the _net_ a circular or square label about the size of a shilling, bearing a distinctly printed number one on each piece, from 1 upwards; and arrange the pieces in any convenient manner by means of wires inserted into a slip of wood; but they must be so placed that the _whole_ can be seen from one point of view, although each piece must be placed so that it is _one inch_ farther from the operator than the next lowest number. Having placed the camera eight or ten feet from the cards, carefully focus to any one of the numbers, 4 or 5 for instance and observe, not that the _number_ is distinct, but that the minute lines or threads of the net are visible: then take a picture, exposing it a very short time, and the threads of the card bearing the number that was most perfectly in focus visually _ought_ to be most distinct; but, if otherwise, that which is most distinct will not only show whether the lens is over or under corrected, but will indicate the _amount_ of error. If under corrected, a lower number will be most distinct; if over corrected, a higher. GEO. SHADBOLT. _Test for Lenses._--I beg to submit to a COUNTRY PRACTITIONER the following very simple test for the coincidence of the chemical and visual foci of an achromatic lens: Take a common hand-bill or other sheet of printed paper, and having stretched it on a board, place it before the lens in an oblique position, so that the plane of the board may make an angle with a vertical plane of about thirty or forty degrees. Bring any line of type about the middle of the sheet into the true visual focus, and take a copy of the sheet by collodion or otherwise. Then, if the line of type focussed upon be reproduced clearly and sharply on the plate, the lens is correct; but if any other line be found sharper than the test one, the foci disagree; and the amount of error will depend on the distance of the two lines of type one from the other on the hand-bill. J. A. MILES. Fakenham, Norfolk.