200 OPTICAL PROJECTION 



amplifier; sometimes a higher power gives better results. 

 Suppose we had a section of human skin, exhibiting both the 

 perspiration glands and ducts, and the touch-bodies which 

 terminate the nerves (though it is not often both could be 

 found in the same section). Probably a power of 1 inch 

 would exhibit the first sufficiently well ; but the -j^-inch 

 amplified to 1,500 diameters would be needed for the touch- 

 bodies, or even a good ^-inch could be used with advantage. 

 So the -^y-inch, amplified or not, would well exhibit the sting 

 of a wasp or bee ; but to exhibit the serrations of the barbed 

 end coarsely, will require a \ or . 



98. The Screen. A sheet is unfit for any work with high 

 powers, unless stretched tight and white-washed. Paper- 

 faced screens, and especially the whitened screens mentioned 

 in 67, are to be preferred, unless a plastered smooth wall 

 can be utilised. For permanent demonstration work it is 

 well worth while to provide the latter; the difference can 

 hardly be conceived without trial. 



For popular subjects I like a screen distance of about 

 25 feet, and up to 50 feet may be used easily, this distance 

 however being chiefly adapted for common objects to be shown 

 in a large hall. High -power ' class- work ' is far better done, 

 on the other hand, with higher powers at shorter distances 

 as short as will allow all of the class to see the screen. With 

 a good -J- homogeneous immersion, or a ^-inch amplified, the 

 cyclosis in Vallisneria can be shown to several hundred people 

 pretty well, with a screen distance of 12 to 20 feet, but the 

 oxy-hydrogen light is not sufficient to attempt such a subject 

 in anything like a large hall. 



Some amount of practice is needed before anyone can use 

 properly even a table microscope ; and it is equally necessary 

 before anyone can produce with the projecting microscope 

 what it is really capable of. Skill will however be rapidly 

 attained ; and when the operator can exhibit a flea, or the 

 proboscis of a blow-fly, 12 to 15 feet long, sharply and 



