THE PARTS OF A LANTERN 31 



slide itself, and these scattered rays also diverge more with 

 the greater distance. This is the chief reason why more light 

 is required in working a long way from the screen ; though 

 light is also perceptibly absorbed by the atmosphere, especially 

 if it is at all a damp night. 



Hence it is an important question, how far this loss of 

 light from long focus must be provided for. Some makers 

 recommend several sets of double -combination objectives of 

 very large diameter, at a cost of 9Z. to 10. apiece ! After 

 investigating the matter carefully, both in theory and practice, 

 I have not the slightest hesitation in saying that such costly 

 lenses are a sheer waste of money, of absolutely no benefit to 

 anyone but the seller of the apparatus. Assuming the back 

 conjugate focus of the condenser to be 4 inches (i.e. about 

 8 inches from the back of the condenser) and the radiant 

 to be |-inch in diameter, nearly all the effective rays can be 

 condensed into an objective of 2-inches diameter, up to 9 

 inches from the face of the front lens of the condenser. 

 This gives a focus for a double combination of, say, 10 inches, 

 within which there is no need of greater diameter, and no 

 benefit from it. Beyond that a 2-inch lens will begin to lose 

 light, but 2J-inches diameter will carry the same result up to 

 13 inches ; and it is only when we reach distances of 15 inches 

 and upwards that 3-inch lenses are of any real advantage. 

 For such long foci, however, the ' double combination ' is quite 

 unnecessary, as with care in selection, single achromatics can 

 be obtained, which will give quite as good images with far less 

 weight and expense ; and perfection should rather be sought 

 in improving this class of lenses, especially in the triplet form 

 described a page or two back ; which will also pass more light 

 than the double form at these long ranges. 



A ' double-combination ' lens of average focus say 6 to 

 8 inches and of most excellent quality, such as will give 

 perfect definition without any stop, is now obtainable for so 

 moderate a sum as II, 10s., including the rack-work mount. 



