5.6 



THE BOOK OF THE LANTERN. 



milled heads. The arrangement is by no means a good 

 one, but it is one of those simple things which have been 

 unheeded by the many, and manufacturers generally have 

 adopted it without perhaps thinking how inconvenient it is 

 in practice. In the first place, the jet is apt to slip either 

 downwards bodily, or to one side or the other by a careless 

 touch of the operator's hand. In either case, such a shift- 

 ing of the light out of the optical axis causes the disc to be 

 darkened until the jet be readjusted, which cannot be 

 easily done without opening the door of the lantern and 

 letting out a flood of light in the darkened room. There is, 

 besides, much trouble in getting the jet central, which could 

 easily be avoided by a more rational arrangement. Mr. 

 Pumphrey, of Birmingham, has devised for the purpose a 

 horizontal and vertical rack motion, very like the same 

 movement which is attached to the mechanical stage 

 of a microscope, but it is somewhat expensive, and 



FIG. 24. 



adds extra weight to the lantern. There is no doubt, 

 at the same time, of its effectiveness and conveni- 



