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. 



