[Illustration] In this combination the front Lens is an Achromatic plano-convex, with, the convex side turned toward the object; and the second, which takes up the rays and refracts them further, is a compound Biconvex Lens; there are therefore in all four distinct glasses concerned in forming the image, which may appear at first to be an unnecessarily complex arrangement. It is found however that a good result cannot be secured by using a single Lens, when a "stop" is inadmissible. By combining two glasses of different curves, the aberrations of one correct those of the other to a great extent, and the field is both flatter and more distinct than in the case of an Achromatic Meniscus employed without a diaphragm. The manufacture of Portrait Lenses is a point of great difficulty, the glasses requiring to be ground with extreme care, in order to avoid _distortion_ of the image: hence the most rapid Portrait Lenses, having large aperture and short focus, are often useless unless purchased of a good maker. _The Variation between the Visual and Actinic Foci in Lenses._--The same causes which produce chromatic aberration in a Lens, tend also to separate the chemical from the visual focus. The violet and indigo rays are more strongly bent in than the yellow, and still more than the red; consequently the focus for each of those colours is at a different point. The following diagram shows this. [Illustration] V represents the focus of the violet ray, Y of the yellow, and E of the red. Hence, as the chemical action corresponds more to the violet, the most marked actinic effect would be produced at V. The luminous portion of the spectrum however is _the yellow_, consequently the visual focus is at Y. Photographers have long recognized this point; and therefore, with ordinary Lenses, not corrected for colour, rules are laid down as to the exact distance which the sensitive plate should be shifted away from the visual focus in order to obtain the greatest amount of distinctness of outline in the image impressed by chemical action. These rules do not apply to the Achromatic Lenses recently described. The coloured rays being in that case bent together again and reunited, the two foci also nearly correspond. By a little further correction to a point higher in the Spectrum, they are made to do so perfectly. SECTION IV. _On the Photographic Action of Coloured Light._ It has already been mentioned in the First Section of this Chapter that certain of the elementary colours of white light, viz. the violet and indigo, are peculiarly active in decomposing the Photographic Salts of Silver; but there are some points of importance relating to the same subject which require a further notice. The term "actinism" (Gr. ἁκτἱς, a ray or flash) has been proposed as convenient to designate the property possessed by light of producing chemical change; the rays to which the effect is especially due being known as actinic rays.