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block (C) in the aperture plate and pass rod (A) through them. It then becomes a simple matter to place the center of the condenser against the pointed end of the rod and thus centering it perfectly with the aperture plate and projection lens.
The plane of the condenser is made parallel to the plane of the aperture by placing disc (D) on the end of the rod and having the face of the condenser touch the disc on its entire circumference. If desired, the mirror may be similarly centered before the condenser is put in place. From the curves shown it is easy to see that if two or more of the component parts of the optical train are slightly out of line a very large loss in screen illumination may result.
The use of wide aperture projection lenses is advocated with incandescent lamps, (Fig. XIV.) The light transmission possible through c^}^" focus lenses of varying diameters is approximately in the following ratio:
Focus
Diameter
Relative Transmission
5K
'/
iK"
^^
5M
'/
MA"
49
5K
'/
1"
64
5K
/
2K"
82
sH
/
2K"
100
sH
/
i%"
121
5K"
3"
144
The use of wic
ie
aperture projection lenses
results
in considerably
greater gam m screen illumination with the tungsten filament lamp than with the arc. In the former, the light rays emerge diverging from the lens and practically its entire aperture is effective. On the other hand, with the arc system we have crossing rays and the outer portions of the lens are not so effective as the center in transmitting the light, /. ^., due to the greater distance between aperture and condenser, and therefore, narrower angle of light with the arc system, a greater proportion of the light flux is passed through the central portion of the objective lens (Fig. XV.)
The color of the light from the incandescent lamp is warmer than that of the arc and generally more pleasing on that account. There are, however, a few conditions where it is desirable to imitate closely the color of the arc. Thus, in film exchanges and studios, a customer who is an arc user may want to select his films with due reference to his own illuminant. In the film studios, where the selection is made, the pictures are generally projected a relatively short distance, 25 or 30 feet, and a fairly small-sized picture shown. Under these conditions the Mazda lamp offers more than a sufficient amount of light.
We may, therefore, sacrifice a small amount in altering its color to conform closely to that of the arc. This may be accomplished by inserting between the condenser and the aperture plate a piece of glass of the proper quality. Such glass is available as a lamp bulb, i. e.^ there are incandescent lamps manufactured for color matching purposes with bulbs made of this very special glass. They are known as Mazda C2 lamps. A good stunt is to obtain an old, burned-out 500 watt PS-40
64