THE UNOFOCAL LENS.
This recently introduced lens, designed by Dr. Rudolph Steinheil, of Munich, in troduces a new principle in the construc tion of photographic objectives ; it is known as the linofocal (Fig. 525). The necessary corrections for astigmatism and Besides the various lenses already re ferred to, there are several others of special make deserving of mention. Among these is the orthoscopie (Fig. 523), designed by Petzval at the same time as his portrait lens, but now little used. It is interesting as being in principle the forerunner of the telephoto lens, having a negative combination at the back, and giving a long focus with a large image. The apochromatic triplet, of which there are several forms, has a central lens of Jena glass which corrects the objective for three colours instead of two ; it is, in addition, free from distortion and flare. These features have made the lens a favourite for three-colour work. The Cooke focussing lens (Fig. 524) possesses an arrangement for focussing by turning a graduated ring on the mount, thus ren dering it unnecessary to look at the ground glass or to move the bellows. It
is on this account peculiarly adapted for use with box hand cameras. The Voigt other aberrations are usually obtained by the use of glasses of great curvature. In the lens under consideration, however, these errors are corrected by glasses of very slight curvature and unusually thin , and transparent. An anastigmat results which is extremely rapid, the same aper ture giving a greater equality of illumina tion and a larger volume of light than in lenses of the old construction. The de finition and covering power are also very fine. A peculiar feature of this lens is that the four elements, two positive and two negative, are all of the same refrac tive index and focal length. Theoretically, this should result in an exact neutralisa tion of the combinations, but the neces sary power is obtained by placing the lenses at slight distances apart.