ENLARGEMENTS I3Y DIRECTED LIGHT 756. Principles of Enlargement by Directed Light. IAA us consider a lens L (Fig. 192) and a drawing on glass N placed perpendic ularly to the optical axis, in such a position that a sharp and enlarged image of N is cast by the lens on the plane P. Let us assume that at C, on the other side of N, there is an achrom atic and aplanatic convergent system, the optical axis of which coincides with that of the lens, and that at S, on the optical axis common to the system, there is a point source of light. of which the image, also reduced to a point, is formed at S' at the centre of the entrance pupil.
Under these ideal conditions, which can never be completely realized in practice, the lens could, without any disadvantage, be of poor quality (or even absent) without affecting the qualities of the image (or shadow) projected at P. The drawing N is, in fact, projected on P from the projection centre S', as the beam issuing from the condenser C is assumed to be aplanatic. Each point of the drawing N is traversed by one ray only, and in particular the intensity of the image formed at P is entirely independent of the size of the stop D. In the case where the image S' of the light-source does not coincide with the centre of the entrance pupil, the sole effect of a gradual closing of the stop D will be to intercept some of the more oblique rays, thus limiting the extent of the part of the plane N of which the image is projected at P. but without affecting the intensity or the sharpness of the central area remaining.
Barring very rare exceptions (reproduction of a line drawing perfectly free from veil), the negative or positive placed at N is more or less a scatterer of light. The whole of the light emerging from such a negative or positive is no longer directed ; a fraction of this light is scat tered and therefore covers the whole aperture of the lens. The combination of condenser and lens does not therefore usually form a single optical system, and therefore its " optics " call for somewhat detailed consideration from the practical standpoint.
Ordinary condensers, made of roughly moulded lenses, are neither achromatic nor aplanatic ;' and, moreover, the light-sources employed for enlargement or projection are never luminous points. There is therefore no point of assembly S' of all the rays issuing from the condenser, but merely a concentration of the beam on a surface, which is greater as the source of light is larger and the aberrations of the condenser are more considerable.
In the conditions which prevail in practice, the case is intermediate between illumination by directed light and illumination by diffused light, but with a marked predominance of directed light. From each point of the image there emerges a cone of rays which more than covers the whole surface of the lens, but with an intensity predominating round the straight line joining the centre of the entrance pupil to the point in question. If the lens is of poor quality, stopping down may then improve the definition a little in the centre of the projected image, reducing the intensity very slightly. But its chief effect is to intercept the best part of the beams forming the image of the margins, only the diffused light being retained, and the conse quence of this is to restrict the field which is uniformly illuminated.
Whereas, under ideal conditions, the illumina tion of the image would depend solely on the intrinsic brilliancy of the light-source used, this is not the case in practice. The brightness of the light is not the sole factor, although its influence is still dominant. For instance, if an incandescent mantle is replaced by another of twice the linear size, furnishing a quadruple illuminating surface with a gas consumption eight times as great, the illumination of the image will not be more than doubled at the most. It is sometimes of advantage, from the point of view of the sharpness of the images, to stop down a light-source of large size in spite of the slight loss of light resulting from the elimination of the portions of the light farthest from the optical axis.