For the purpose of comparison the following table gives some particulars of 300-watt projec tion lamps of a reputable make As the illuminating plane formed by the juxtaposition of the windings emits light from both its surfaces, a sphero-concave mirror 1 is placed behind the lamp to prevent waste of light from the rear surface. This mirror is mounted on an adjustable support and can be brought into a position such that the images of the windings are formed in the space between two adjacent Under these condi tions, the light that would have been lost is sent back to the condenser, the whole of the filament and of its image thus forming an almost continuous luminous surface. When the re It may be added that with incandescent lamps the adjustment once made remains un altered, whereas arc lamps, even with the usual automatic types, require frequent adjustment.
759. Adjusting the Position of the Light. The adjustment of the position of the light must be preceded by the projection of the image to the required size and in rough focus (§ 768). After this preliminary adjustment has been effected, the negative carrier is taken out and the light is centred by examining the illuminated disc in the plane that will be occupied by the sensitive surface.
flector is properly adjusted, the light-efficiency of the lamp is increased by 50 to 70 per cent. 3 If the centring is perfect, this disc will be uniformly illuminated. As a rule this will not be the case, faults in centring revealing them selves by characteristic shadows.
If the light-source is centred on the optical axis, but is at an incorrect distance from the condenser, the illuminated disc appears bounded by a darker zone which is bluish (light too near the condenser) or reddish (light too far from the condenser). Let us suppose (Fig. 193) that the condenser and the lens occupy respectively the positions C and L, and that the light-source S is in such a position that its image 5', produced by the marginal portions of the condenser, is formed a little in front of the lens. Then all the light transmitted by the condenser will be utilized by the lens, and the latter will produce on the easel a fairly sharp and uniformly illum inated image of the condenser. Let us suppose that, for an enlargement on a smaller scale, the lens is moved farther away from the negative carrier so as to occupy the position L'. It is obvious that we shall restore the former condi tion by bringing the light-source S nearer to the condenser C until the image S' is close to L'. If this is not done, i.e. if the light is left too far from the condenser, the marginal parts 0' of the disc projected on the easel are much less illuminated, being lighted only by the rays least refracted, that is, the red rays.
If the light is not centred on the optical axis common to the condenser and the lens (Fig. 194), decentred shadows appear, reddish in the posi to about 5 in. from the condenser, that is a shift of 4 in. when passing from one position to the other.' 760. Enlarging Lanterns. The usual types of enlarging lanterns are described in the cata logues of the principal dealers, so that it is only necessary to make a few remarks and to indicate the conditions to be observed in order that the adjustments described in the preceding para graph shall be possible.
According to the maximum size of the nega tive to be enlarged, it is preferable to use a condenser of the diameter shown in the following table tion 0 or bluish in the position according as the lens is in the position L, or i.e. near the image 5, of the source produced by the marginal rays, or near the image of the source formed by the central rays. (In the figure the
spherical aberration of the condenser has been greatly exaggerated.) It may therefore be said that in all cases a blue shadow indicates that the light must be moved farther from the condenser, whilst a red shadow is a sign that the light must be brought closer. Finally, any unsymmetrical form of the illumination of the disc shows that it is necessary to shift the light transversely in the direction indicated by the centre of the shadow.
If, for instance, the condenser has a focal length of 5 in. and the negative is at 2/5th in. from the plane surface (of the condenser) nearest to it, and the degree of enlargement is first 2, and then 1o, in each case with a lens of 6 in. focal length (the ultra-nodal distances from the negative to the lens being 6:_-; in. and 9 in., the ultra-nodal distances from lens to condenser being about 7,.; in. and Io in., with a condenser of a total thickness of about I in.), then it will be necessary for the light-source to be brought to a distance of about 91 in. and then The lens must not be chosen of too short a focal length, especially if it is wished to enlarge on a large scale, for it may be then impossible to project the image of the light-source near to the entrance pupil of the lens; insufficient length of the lamp house may make it impossible to move the light far enough back. Besides this, if the light is at a distance from the condenser more than twice the focal length of the latter, the light-rays converge on leaving the first glass, so that the condenser does not illuminate so large a circle in the plane of the negative which is being enlarged. On the other hand, a lens of too great a focal length renders it neces sary to have a room of considerable length (§ 6a). In practice it is best to select a lens of which the focal length is at least equal to that of the condenser and does not exceed it by more than 30 per cent.
It should be noted that a lens intended for enlarging by directed light should have an iris diaphragm of metal, as leaves of ebonite may be deformed or burned (§ 72).
The lamp-house should be large enough to accommodate modern high-power illuminating apparatus. It must be well ventilated, and the ventilation holes must be perfectly light-trapped. The run-way of the lamp support must be enough to allow of all the variations in adjust ment that may be required for various degrees of enlargement. Doors, with observation holes fitted with ruby glass for watching the light, must give free access to the various regulating devices and to the condenser. A slot should be provided for a sheet of ground glass. This is best placed against the surface of the condenser farthest from the light, and enables the lantern to be used for enlarging with diffused light if desired. The position of this ground glass must be far enough from the negative for its structure not to appear in the enlargement.
The negative carrier should slide easily into its slot, but without play. Sometimes the nega tive stage is movable in relation to the condenser, to which it is then connected by a bellows. This is of value in cases where it is desired to enlarge negatives smaller than the normal size, because they can be placed in a position where the converging beam of light from the condenser is of a smaller section, and thus affords a greater concentration of light." It is a good thing for the negative carrier to be capable of being de-centred, and it is desirable that the lantern should include no other part capable of being de-centred, at any rate when the lantern is being used with directed light.