8CIOPTI CON MANUAL. 133 



of the disk on the screen is about one-quarter of the distance 

 away ; so that, to get a 20-foot disk, the instrument must 

 be 80 feet away, a 15-foot disk 60 feet away, etc. The back 

 cell of the objective in more common use may be removed, 

 which gives nearly the same result as to spaces, but the 

 margin of the image will lack distinctness, and will be 

 tinged with yellow. 



There is some call for an objective of shorter focus, to be 

 used behind the screen, where there is but little room. A 

 corrected lens can be supplied, giving an image about the 

 measure of the distance, but with less perfect definition. 

 Non-achromatic objectives are easily made with short focus, 

 because they are not combined with the flint-glass dispers- 

 ing lenses to correct the aberrations. 



Some more of the light may be rendered available by 

 placing it, say two inches from the face of the condenser ; 

 but much of the gain is lost by its falling on the face of the 

 lens at so near the angle of total reflection. A practical 

 objection to placing the light so near, is the danger of crack- 

 ing the glass by heat. 



Spacing, and other details herein given, are for the Sciop- 

 ticon operator, and may not apply in all respects to other 

 apparatus. As to oil-light lanterns, it seems enough to 

 have described the Oil-light Sciopticon. Lime light, how : 

 ever, being more concentrated, and being controlled by 

 more artificial conditions, allows a greater variety of ar- 

 rangements, and of rival forms of apparatus to compete with 

 each other for public favor. So the Oil-light Sciopticon 

 monopolizes a lion's share of our space, for lack of a worthy 

 rival, and the Lime-light Sciopticon is exclusive, because 

 the forms of like apparatus are too numerous here to par- 

 ticularize. 



