THE BOOK OF THE LANTERN. 19 



to this imaginary picture upon our screen, we now see that 

 it is complete. It is no longer the central fragment of a 

 design, but covers the sheet, and is equally illuminated. 

 We can at once see the reason for this welcome change by 

 looking once more at our revised diagram, fig. 11. The 

 rays of light instead of being wasted in illuminating the 

 inside of the lantern box, are refracted by the lens which 

 we have introduced, and are bent towards the objective. 



In my diagrams, for the sake of simplicity, I have 

 represented each lens as consisting of a single piece of 

 glass of plano-convex form. Such lenses are found in toy 

 lanterns of the cheapest kind, but are, as might be ex- 

 pected, extremely faulty in performance. In the enlarged 

 picture, they give, owing to their total want of correction, 

 badly-defined margins, curved lines, and fringes of colour. 



Having now seen the purpose fulfilled by the condensing 

 lens of the optical lantern, let us further consider its best 

 form, and let me at once correct an error into which a 

 purchaser is likely to fall. I have sometimes heard the 

 possessor of a lantern speak somewhat boastingly of his 

 instrument as one with 5, or perhaps 6 -inch condensers, 

 the more ordinary size being 4 inches, and often only 

 3 1 inches. For lantern projection, any size over 4 inches 

 is a positive disadvantage, and instead of representing a 

 gain, means really a great loss of light. The reason for 

 this is readily seen. What may be called the standard size 

 for a lantern picture is 3 inches in diameter (the entire 

 slide with its margin measuring 3J inches). If the picture 

 be framed in a circular 3-inch mount, a 3J-inch condenser 

 will amply illuminate it. If, however, the orifice of the 



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