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with in practice. Beyond the point of maximum concentration this beam does diverge as the paper shows.
If the condenser has spherical aberration the rays through the center will image at, or as we say, "converge to," a point beyond the point of maximum concentration. At this imaging point of the central rays the rays through the outer part of the condenser have already converged to an image and now diverging form the boundary of the diverging beam. Thus we see that the beam is diverging beyond the point of maximum concentration, as the name of this point indicates, some of the rays are still converging.
This misunderstanding simply illustrates the necessity already pointed out in this meeting of care being exercised to use terms taken over from the older branches of optics in their previous meaning where this is possible.
The question as to whether the point of maximum concentration should be at the aperture plate or at the objective is a very different matter. To consider this we must advance but one theory and that amply proved experimentally. That theory is that the maximum light at any point from any source of uniform brilliancy through any opening has been obtained when the entire apparent area of that opening is covered by the source of its image. This, of course, leaves out of account losses due to the reflection from lens and mirror surfaces and absorption in their materials. This means that if at any point beyond the aperture plate every point of the film has a bright spot of the condenser behind it, at this point we have as much light through the film as is possible to obtain with a source of this intrinsic brilliancy. If this is true for every point on the objective lens, we are supplying to this lens all the light it can use, for each of its points has the maximum light available. If there are any points alongside the objective for which this condition obtains, then the lens is not using all the light the rest of the system is offering it. Let us be sure to remember, however, that the condition is that behind all points of the film there shall be an illuminated area.
If we consider a vertical section through a diagonal of the aperture plate and the center of the objective, and if we draw lines from the top and bottom of the objective opening to the bottom and top of the aperture respectively, and extend these lines in both directions, they obviously indicate the cone that must be filled with light in order to have our conditions of maximum brilliancy satisfied, and it does not in the least matter from the standpoint of quality of light through the objective how far from the aperture plate the source or condenser is placed. The only requirement is that the base of this cone as seen from all points of the objective shall have at every point the brilliancy of the source. If we could use a source large enough to cover this base at any point beyond the aperture, no combination of mirrors and lenses with this source would give us one iota more light on the screen. By the use of a condenser, however, we can cut down the necessary size of our source and in so doing decrease our power consumption.
The object of the condenser is then simply to make the source seem larger, so large in fact for best illumination that it covers the
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