34 OPTICAL PROJECTION 



length, which is best as a single tube, but may be a double 

 sliding one as shown in the figure. In the first case the whole 

 .7 ^ is perfectly rigid, and per- 



fectly simple ; and lengthening 

 tubes may either be ordered 

 with the original apparatus, 



r~ or, if suddenly called for by 



FIG 21 Ada ter some extraordinary occasion, 



could be fitted in all large 



towns in a few hours, by any working optician worthy of the 

 name. 



This method of construction is being gradually adopted 

 by most of the lantern-makers who really have much experi- 

 ence with the instrument. 



CHAPTER III 



THE RADIANT 



THE qualities desired in the radiant are brilliance and 

 \\hiteness of light, and that this light be concentrated into as 

 small a space as possible. The perfect radiant would be an 

 intensely luminous point, which would give the most equal 

 illumination and the best definition. The radiants used in 

 practice by no means come up to this ideal. They comprise 

 (1) fatty oil lamps, (2) petroleum oil lamps, (8) gas-burners, 

 (4) the lime-light in its various forms, and (5) the electric 

 light. 



19. Fatty Oil Lamps. Except in small toy lanterns, it 

 is seldom we now find these lamps employed ; but for many 

 years they were the only radiants used at all, even in public 

 exhibitions of dissolving views. With hand-painted trans- 

 parent slides they did good work, too ; giving usually discs of 

 eight feet diameter on transparent screens. They will not 



