The Art of projection and complete magic lantern ... - Yesterday Image

62Examples.—An Sin. focus lens at a distance of 35ft will give a discof 13ft. I in. To produce a disc of 12ft. with a lens of loin. focus, thelantern and screen must be separated by 4ofc. To produce a disc of15ft. at a distance of 45ft. will require a lens of gin. focus.THE OBJECTIVE MOUNT.Fig. 36The lenses themselves are always mounted in tubes as Fig. A. and B.,but these again must be mounted or placed in a jacket with a pinionworking on a rack which is screwed on to lens tubes A. B., being rackedin or out by the milled heads, when the lens tube is so mounted itbecomes a fixture in the jacket or mount, and is the best arrangementfor solidity.To save weight and slightly reduce the expense, another plan isvery servicable, when lenses of various foci are intended to be used, thatis, to have the jacket or mount so constructed that each combination inits tube will slide into the mount, usually from the front, or screwed infrom the back. The sliding plan seems to commend itself moregenerally, the tubes should fit into the mount somewhat tightly, especiallyfortwo or more lanterns.The lens mount Fig. 36., is fitted with a double pinion, enablingfocussing to be worked from either side, and is fitted with a diaphragmslot in the front to enable coloured glasses to be introduced. The capor flashing shutter is shown in the illustration on the wrong side, itshould open over to the right, otherwise the hand will be in front of thelens when uncovering. When setting up a lantern, rack the lens outhalf-way (as shown in engraving) focus roughly with the front tube intowhich it is screwed, and give the final definition with the pinion, neverrack the pinion out to its full extent, it is sure to cause a downwardtendency, although only very slight.