757. Condensers. The condenser of old-time " magic lanterns " was a half-sphere of glass. The condenser has been slightly improved by being formed of two identical piano-convex lenses the poles of which are almost in contact.' Occasionally condensers are used of which the components, named in their order, starting from the light-source, are a converging meniscus (concave surface turned to the light) and a bi convex lens, or a converging meniscus (turned as in the preceding case) followed by the two piano-convex lenses of an ordinary condenser. As the focal length of this triple condenser can be shorter than that of an ordinary condenser of the same diameter, the light-source can be brought nearer, thus utilizing a larger portion of the light emitted. x We may also mention the recent development of aplanatic condensers, including at least one surface of non-spherical curvature, up to diameters of 25 in. (L. V. Foster, 1926), and of condensers with zonal aberrations adjusted so as to attenuate the falling off of light from the centre to the margins of the projected image (Zeiss, 1931).
Condenser lenses, being always comparatively thick 2 and made of glasses the transparency of which is not perfect, absorb an appreciable fraction of the rays which reach them and thus gradually become heated, sometimes attaining high temperatures with a resultant risk of fracture in case of sudden cooling by a current of air. 3 The usual mounting of condensers, a tube in which the lenses are kept at their normal separa tion by a cylindrical sleeve and held firm by screwed rings, could very easily be improved. The screw threads are ridiculously fine in regard to their diameter, so that putting together a condenser after cleaning is a veritable feat of patience. Also, while ventilation holes are pro vided to allow of the expansion of air without interior pressure, they are more or less closed by the sleeve (on the lantern) into which the condenser is fitted, so that they do not allow the renewal of the air within. Various improve ments have been devised in the construction of condensers, but these advances have not been applied. For example, segments have been cut out of the lenses permitting the free circulation of air between the glasses and their mount (L. Turillon, 1902) ; or the screw mount has been replaced by a mount with elastic claws, both permitting the free circulation of air and sim plifying the work of taking to pieces and putting together again,' as well as the replacement of lenses, in the diameter of which there is always a certain latitude (G. M. Coissac, 1905).
758. Light-sources. The first enlarging lan terns were fitted with paraffin oil lamps with round burners, or, where great intensity was required, with arc lamps. Lights of intermediate power subsequently came into use, particularly incandescent and acetylene burners. In all cases where electric current 3 is available the invariable practice to-day is to use the arc or an incandescent lamp of the special type made for projection In an arc lamp run on continuous current, the region of maximum brilliancy is the crater hollowed at the tip of the positive carbon. This crater must be situated
on the optical axis and turned towards the condenser. In an arc lamp run on alternating current the two carbons play the same role alternately, and as they cannot both occupy the optimum position, the efficiency is neces sarily bad during one of the cycles, that is, during half the time of working. On the other hand, electric current is usually supplied at 230 volts, and the usual arcs only absorb 45 volts. While, with alternating current, there is the resource of lowering the voltage at will with a transformer without losing much power, there is, with continuous current, no means except absorbing the excess voltage in a resist ance where the electrical energy is converted into heat.
The efficiency of the arc lamp is thus poor with continuous current (about $o per cent of the energy consumed being lost in the resist ances), while its light efficiency is mediocre with alternating current. Owing to the great im provements made since 1917 in the manufacture of the incandescent lamps specially intended for projection work, they have practically sup planted the arc everywhere, the latter being now used only in " still " projection lanterns and cinematographic projectors where very high luminous power is required.
The incandescent lamps specially designed for projection work differ from the ordinary lamps for lighting in the arrangement of the filament, which is formed of several helical windings 2 arranged parallel to each other in the same mean plane. There are two windings for lamps of 12-16 volts, and four for those of 25-220 volts. For a given consumption it is more advantageous to choose a lamp of low voltage and large amperage, for the larger filament can be brought to a higher temperature, which enhances the luminous and, even more, the actinic efficiency. Furthermore, the helical windings can be closer without risk of arcing between adjacent windings. This reduces the illuminat ing surface and thus improves the action of the condenser. Some of these lamps are made to work vertically, with the holder beneath, while others are intended for use horizontally, accord ing to the arrangements of the lamp house,' and it is essential that the lamps be used in the position for which they are designed. The normal working condition of these lamps corre sponds to a considerable over-running of ordin ary lamps, and their life is usually limited to roc) hours. 2 Care must be taken not to over-run them farther, and it is well to have instruments enabling the current to be measured and regulated With continuous current it is not possible, except with complicated arrangements or con siderable loss of electrical energy, to use lamps of a voltage different from that of the supply or, at any rate, of slightly lower With alternating current it is best from all points of view to install a transformer, bringing, for in stance, the difference of potential to 22 volts for a 2o-volt lamp in series with an adjustable resistance.