THE BOOK OF THE LANTERN. 25 



effect whatever upon the performance of the lens, whether 

 used with a camera, or as a lantern objective, supposing it 

 to be suitable to lantern work in other respects. But 

 transfer the bubble from the objective to the condenser, and 

 it at once constitutes a real eyesore, which will be terribly 

 magnified on the screen. For this reason lantern owners 

 should take the greatest care to prevent their condensers 

 becoming scratched, for a mark hardly visible on the glass 

 will become fatally apparent on the screen. 



It will be seen, from what has gone before, that the duty 

 of the condensing lens is to take up and utilise as large a 

 bundle of light rays as is practicable, and with those rays to 

 brightly illuminate the whole of the picture or slide placed 

 against it. The other part of the optical system of the 

 lantern is the objective lens, which is destined to 

 form a magnified image of that picture or slide. To this 

 important part of the apparatus I must now turn the 

 reader's attention. 



As the duty of the condenser is to give the greatest 

 amount of illumination to the lantern picture or slide, so 

 the province of the objective lens is to form as perfect as 

 possible a magnified image of that picture upon the screen 

 or sheet placed for its reception. Toy lanterns are fre- 

 quently fitted with a double convex, the worst form of all, 

 or with a plain convex lens, which is little better. With 

 regard to lanterns of more pretension, we find that different 

 makers adopt different forms and combinations for their 

 objectives. Some use a couple of plano-convex achromatic 

 lenses, in conjunction with a stop or diaphragm, the flat 

 sides of the lenses being next the light ; a very 



