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32 OPTICAL PROJECTION I have seen such lenses most carefully tested against the most expensive of similar focus, without any perceptible difference being discernible. 17. Testing Lenses.—It is no use to ' test' a lens upon an ordinary slide, for anything beyond definition and flatness of field. These points can be seen by the image of any good photograph, but the ' figure ' may be quite distorted. The very best test is a cushion-slide covered all over, to the corners, by lines of type sharply photographed, or by black lines ruled in squares. A lens which gives an image of this sensibly alike in focus all over, and the lines or squares straight, especially towards the corners of the cushion, has every needful quality for projection, though the photographer must, of course, look for his special requirements in addition. 18. The Objective Mount,—A lantern only meant to be used at one focus needs no consideration in this respect; into the nozzle of the lantern will slide stiffly a tube of the proper length, into the end of which will screw a rack-and-pinion mount carrying the objective. Practically, all such lanterns are made alike. The travelling exhibitor, however, often needs a very wide range of focus, perhaps from 4J inches up to 20 inches, or more. To give him this, three methods have been adopted. The first is to fit the front of the lantern with what are termed telescopic draws, as shown in the tri-unial lantern on p. 119, into the front of which the objective rack-mount screws. No plan has been more usual than this; and if the draws are of first-class workmanship, and fit tightly, it works well for a greater or less time. It is, however, very difficult to pull such draws out, and at least one leading lantern optician has found it desirable to insert a pair of strong metal handles into the front draw, in order to give more strength to the pull. Sooner or later, however, the draws are apt to wear a little loose, and then the front end sinks out of the true optic axis under the weight of the objective.