66 OPTICAL PROJECTION 



were it not for its cost, which is more than double that of the 

 preceding. 



Mr. Wood has further applied to his jet a shield or jacket 

 of copper surrounding the lime at a little distance, except a 

 space cut out in front for the play of the jet and for radiation, 

 which he strongly recommends as increasing the light (by 

 confining the heat), keeping the lantern cool, and avoiding 

 cracking of the lenses. There does appear to me a perceptible 

 gain in the two latter points ; but I am unable to trace the 

 least gain in light, and the copper hinders the state of the 

 lime from being seen through the side of the lantern. The 

 shield can be applied to any jet by such as desire it. 



33. Attachment and Fixing of Jets. The figures already 

 given show the principal methods of adjusting and fixing jets in 

 the lantern. Far the most common is a tray of thin sheet-iron 

 or stout tin, as shown in fig. 28, from the back end of which 

 stands up a steel rod, over which the socket of the jet slips 

 and can be secured in any position by a screw. The edges of 

 the tray slide backwards and forwards in guides, so as to draw 

 back the jet to any distance from the condensers. The sliding 

 adjustment has to be rather loose, and with the view of 

 obtaining more accurate motion in the line of the optic axis, 

 many opticians mount the rod on a wooden board of 

 mahogany, as in fig. 81, and I have also seen a jet mounted 

 to slide along tubes. I find that wood, however carefully 

 seasoned, is very uncertain in its behaviour in any but a single 

 or bottom lantern, owing to the heat ; hence, metal plates 

 with dove-tailed edges are better. My own opinion is, how- 

 ever, that the slight amount of freedom or ' play ' in the 

 ordinary tray is, for ordinary work, an advantage, facilitating 

 ready adjustment of the light. 



I also prefer myself, for common exhibition work, to have 

 no fixing of the jet whatever, beyond one screw to pinch on 

 the rod. Many think differently, and various opticians make 

 their jets with a fork-piece projecting sideways from the front 



