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The Optical Magio Lantern Journal and Photographic Hnlargar.
The Magic Lantern : Its Construction, Illumination, Optics & Uses.
CHAPTER XI.—THE POSITION OF THE LIGHT.
Wirt regard to the relative position of the source of illumination to the condenser, it should be remembered that the light must be placed, not only at the proper distance from it, but also in the exact axis of the optical combination, in order to project a properly illuminated disc upon the screen. By properly illuminated is understood that the circle of light shall be of the same intensity throughout, and quite free from the least trace of shadow, either at the centre or sides ; and unless the light is in its proper place, dark patches will appear on the screen according to the relative displacement.
If the patch appears on the right hand side of the disc, as at Fig. 38, the jet will be found to be
Fic. 3S, too much to the left hand side of the condenser, and it must be gradually moved towards the other side. Should the shadow, however, appear as in Fig. 39, the jet must be moved in the opposite
Fic. 39.
direction until the side shadows disappear from the sides of the disc.
On the same principle, if the flame is below the centre of the combination, a shadow, as at Fig. 40, will be cast ; and if too high, a patch will
Fic. 40, appear upon the screen in the position shown at Fig. 41.
Fic. 41.
The aforementioned shadows, which are of a bluish-grey colour, will only appear as stated if the light be at, or about, the correct distance from the condenser. Patches of a different shape will be shown if the distance of the light from the condenser is either too great or too little.
A shadow of a circular fringy character, in the locality shown in Fig. 42, denotes that the light
Fic. 42.
must be pushed further forward ; whilst a central dark spot, as at Fig. 43, indicates that the flame is