r8o OPTICAL PROJECTION 



place upon the screen. No such instrument as even the 

 attachment described in Chapter XL, which was itself founded 

 on the more complete instrument here to be described, was 

 in fact then obtainable. 



A brief examination of the instruments then accessible 

 proved that the meagre results proceeded from want of appre- 

 ciation of the conditions. The oxy-hydrogen radiant is a 

 luminous surface as large as the thumb-nail. The light from 

 this can never be condensed into a point, but only into an 

 image of that surface, whose size depends upon the focus 

 employed. With high powers, we cannot condense all the 

 light upon objects, say even of 1 mm. diameter. The utmost 

 concentration we can obtain, is a simple function of the relative 

 foci of the original lantern condenser, and the substage con- 

 denser. But if we use for the latter a large lens of very short 

 focus, then we get too high an angle, and most of the light 

 crossing in and diverging from the object, never passes 

 through the objective. If, on the other hand, we use a very 

 small lens, most of the light never gets through this lens. 

 Hence, each secondary, or substage condenser, must be specially 

 constructed for powers of a certain range only, in focus and 

 angle. Except for low powers, each substage condenser is 

 almost necessarily composed of two lenses ; one of fairly 

 large size, to take up and bring down all the cone of light, 

 the other, to still further condense that light on the object, 

 without employing an angle too great for the objective. There 

 was further to be studied perfect protection from the heat, 

 which is very great ; and simplicity of parts and manipulation, 

 without which, work in the dark cannot be effectively per- 

 formed. 



The matter thus presented itself to me from the first 

 simply as one requiring practical adjustment in these points. 

 All the principles concerned had, in fact, been pointed out by 

 the Eev. W. Kingsley so far back as 1852 ; so clearly that I 

 was quite surprised to be unable, after some search, to find 



