CONDENSER In optical projection the condensing lenses cause the rays of light emitted by the lamp or jet to pierce the transparency from all points, the rays being then transmitted to the objective or focusing lens. Diagram A shows the general optical system by which projection is accom plished, E being the illuminant, r condenser, G transparency or slide, and H objective or focus ing lens. The condenser shown is the one com monly used. Light rays, unless intercepted, always travel in straight lines and, as indi cated at E, in straight lines from their source. This holds good, no matter whether the illu minant be oil, limelight, acetylene, or electric light. For the purposes of optical projection, it is necessary to collect a large angle of these rays, transmit them through the slide, and pass them on to the projecting objective, by means of which a large image is brought to a focus upon the screen placed at some distance beyond.
In cases where the illuminant may be regarded as a point, such as with the limelight or electric arc, many experts consider that the Herschel form of condenser B possesses advantages. Many years ago two lenses of somewhat long focus and shaped as at C, were employed, but this form has long been obsolete. The forms shown at A, B, and,C are capable of collecting a fairly large cone of rays ; the lens next the light serves as the collecting lens and. for, transmitting the rays to the second lens, which bends and converges them towards the objective, as at A. The dis
tance of the illuminant from the condenser governs the angle at which the rays are sent on towards the objective. The triple form of con denser D possesses many advantages for micro scopic and other scientific work, inasmuch as it collects a greater angle of light.
In the best condensers the lenses are ground to a sharp edge. The lens next to the illuminant unavoidably gets very hot, and hence should be mounted loosely in the cell, for if at all tight it will, in expanding with the heat,, become cell bound, and a crack will probably result. One method of preventing this is' to provide the mounts with three or four spring clips or spring rebates; which allow of the expansion of the glass and also of ventilation. The intense heat arising from powerful arc lamps, such as are necessary in kinematograph projection work, tf•nriP to overheat the lenses in a very short time, and the.anger of breakage is therefore increased. Several makers, recognising this fact, have pro vided means for the ready removal of the con densers from their cells, so that fresh ones may be substituted at intervals during a display, and this without serious interruption. When using all forms of condensers, sudden cold draughts must be guarded against, or cracking will almost certainly occur.
The bull's-eye condenser, as used in photo micrography, is described under its own heading.