LENSES. 



Fig 



sometimes necessary, 



MOUNTINGS FOR LENSES. 



Lenses may be purchased already mounted in any 

 desirable way, but generally the mounting costs as much 

 or more than the lens. What is 

 needed is a fixture that will hold 

 the lens at a proper height, and 

 has a considerable latitude of 

 movement in every direction. 

 For many purposes an unmount- 

 ed lens can be held in the pinch 

 of a common retort-holder as in 

 Fig. 14. 



It is often convenient, and 

 to have the lens so mounted as 

 to cut off light that would otherwise pass by its edge 

 and mar the effect upon the screen. To do this it is 

 only needful to cut a hole the size of the lens, in a 

 square piece of board (Fig. 15) having the proper 

 size and thickness, and fasten 

 the lens in it with small brads 

 or triangular pieces of tin or 

 zinc, such as glaziers use for fast- 

 ening window-glass in the sash. 

 The board should be thicker than 

 the lens, and the latter should be 

 Fig. 15. so sun k into it that its surface will 



not touch the table when lying upon it. This will pre- 

 vent it from getting scratched. Thus mounted, the 

 board may be held in the retort-holder as before. 



The smaller lenses, having a focal length of not more 

 than an inch or two, had better be bought already 

 mounted for carrying in the pocket, such as botanical 

 glasses and linen provers, or, if it can be afforded, get 

 one of Zentmayer's gas microscope objectives. 



