160 PRACTICAL PHOTOMICROGRAPHY. 



The axis of the lantern optical system may be horizontal, 

 and the lantern may be raised so that this axis " produced" 

 falls upon the centre of the s creen ; or the lantern may be 

 placed on an ordinary table or stand tilted upwards ; and the 

 screen tilted the opposite way until it is perpendicular to the 

 optical axis of the lantern. 



The lantern slides are held in position in the lantern by a 

 device known as a " carrier." The simpler this carrier is the 

 better, provided that the slides can be easily and certainly 

 centred. ~No carrier for our purpose is likely to excel the old 

 "Chad wick," well known in England, at least ; but one of the 

 mechanically centering ones may be used. 



The projection lens should have a focal length suitable to 

 the size of disc required and the distance from lantern to 

 screen. 



/S=Size of opening in slide in inches. 



Z>=Diameter of disc in feet. 



Z=Distance of lantern from screen in feet. 

 length of projection lens in inches. 



LxS 



(From "The Magic Lantern Manual," by W. I. Chad wick. 

 London : Warne & Co.) 



A good light is obtained more by careful adjustment of the 

 proportions of the gases than by heavy pressure. It is, of 

 course, necessary to accurately centre the various parts of the 

 optical system, and to get the radiant as nearly as may be in 

 the exact focus of the condenser. 



The limes should be always kept in a dry or air tight 

 receptacle, and it is well to heat or even bake them for a short 

 time before use. 



If bag and pressure boards are used the latter must have a 

 free fall, and no person should be allowed to touch the bag 

 when the gas is alight. The weights must be of such shape, 

 or so fixed, as to have no chance of falling off. 



If any hitch should occur the oxygen is to be turned off 

 first ; on lighting up at first the hydrogen is always to be 



