THE PARTS OF A LANTERN 33 



A second plan consists in fitting separate draws, each, 

 with rack-and-pinion. This is known as the ' triple- rack ' 

 front, and the racks remove the difficulty in pulling the front 

 out for use. Of the rest I am not so sure, my experience of 

 racked tubes in general being, that with regular use stiff ones 

 either wear loose, or * grind ' and set fast. I have, however, 

 had no actual experience of these fronts themselves, and have 

 seen a letter which, after four seasons' use, speaks of them 

 in very high terms. 



The third plan, which I regard as the only good one, is 

 rigid, simple, and the cheapest and lightest of all. It consists 

 in somewhat lengthening the lantern nozzle A and tube B 

 sliding into it, so as to give there alone an adjustment of 





Pia. 20. Lantern Front 



three inches. Into the outer end of the tube B screws a 

 diminishing screw- collar or adapter c, into which again, for 

 all moderate foci, is screwed the rack-work mount D. This 

 racked mount does not, however, carry the objective directly, 

 but is only a casing, into which the different objectives, 

 mounted in simple smooth tubes, are fitted to slide. This 

 gives another sliding adjustment of three inches. Thus the 

 original mount alone, even if the shortest and longest focus 

 are double combinations, will give a range of focus from six 

 to twelve inches, which will cover the needs of the great mass 

 of exhibitors ; with a single achromatic lens it practically 

 becomes over thirteen inches. If, however, very long focus 

 is required, the adapter c is removed, and in its place is 

 screwed a lengthening adapter (fig. 21) of any required 



D 



