*Scientific Memoirs, vol. iii., p. 535.
We commonly hear of a lens being slow or quick ; this is purely accidental, arising entirely from the uncertainty in which all our optical instrument-makers remain as to the relation of the chemical and luminous forces to each other.
If the lenticular correction does not extend to the point of bringing the rays beyond the violet, upon the field of vision, the lens will be slow in action, because the light rays interfere, as is explained in a previous page; but if it is truly diactinic it will be a quick lens.
For portraiture, and all purposes requiring great distinctness of outline and rapidity of operation, two achromatic lenses are usually employed. By this arrangement the focal distance is diminished ; the image is much reduced in size, but then it is concentrated in every respect, and hence improved in all the necessary particulars. These lenses are, however, still open to the objection that they produce some distortion, which is only to be avoided by greatly reducing the size of the aperture through which the light falls on the lens, and this necessarily involves increased sensibility in the preparations we employ. The distortion is not to the extent which has been represented, but it may, by careful examination, be discovered in the finest photographic portraits to a greater or less extent.