242 SEC. 7. LIGHT. 



possess what is called "depth of focus." A perfectly corrected lens, of 

 sufficient angular aperture or rapidity of action to make portraiture possible, 

 has no " depth of focus." An instrument, possessing a certain residual 

 amount of spherical aberration, solves the difficulty to some extent. Un- 

 fortunately, however, a lens so constructed works at its best only for a given 

 size of image or distance of object. If this be removed to a greater distance, 

 as for a smaller image, the aberration is in excess, and if placed nearer, the 

 converse obtains. 



The lens now to be described surmounts this difficulty. Its aberrations, 

 both spherical and chromatic, are perfectly corrected when used intact, or as 

 sent out by the maker ; and by the simple turn of a screw, separating the 

 component elements of the posterior combination, this correction can be 

 modified at will, i.e. positive spherical aberration, or " depth of focus," is 

 obtained, proportionate in amount to the separation of the posterior lenses. 



The front combination, composed of a double convex crown, and a double 

 concave flint glass lens, is cemented, to prevent loss of light from reflection. 

 The convex or crown lens occupies the exterior position. At a distance 

 equal to the diameter of the front is placed the back combination ; an un- 

 cemented compound of the same diameter as the front, but of greater focal 

 length, i.e. as 2:3. The crown element of the back combination is a 

 meniscus with its concave surface facing the front combination. The flint is 

 concavo-convex, the convex side external ; this lens is mounted in a cell, the 

 screw of which affords the means of approach to, or separation from, the com- 

 panion crown lens. An index and division registers the amount of separation. 



The converging cone of rays refracted by the front is incident upon the 

 crown element of the back combination ; on emergence it is more strongly 

 convergent when it meets the concave surface of the posterior flint ions. / It 

 is evident that any alteration in position or distance at once reduces or in- 

 creases its effective diameter, or, in other words, its aberration (j/ 2 ). The 

 aberrations being perfectly corrected when the posterior flint lens is screwed 

 home, the index pointing zero, the smallest amount of unscrewing or increased 

 separation at once introduces positive spherical aberration. 



In Petzval's lens the position of the lenses composing the back combination 

 is the reverse of the above ; i.e. the flint, or convexo-concave element, faces the 

 front combination, the rays when refracted by it are parallel or nearly so, 

 and any alteration of distance of the companion crown lens is without effect 

 upon the state of aberration of the objective as a whole. 



The portrait lens constructed on the new formula is free from disturbing 

 reflex images ; it produces pictures approximately free from distortion, and 

 illuminates a larger angle of field. It is made of three descriptions : viz. the A 



series, aperture &?*, and D -= -L. 



4 o u 



3. A single combination landscape lens o 2" diameter and 12" focal length 

 for pictures on plates 12" x 10". It is composed of three lenses, two of which 

 are of crown glass, but of different optical properties, and between these two 

 is the correcting flint glass lens. The crown lenses are deep menisci, ratio of 

 foci of front to back as 1:3; the flint is concavo-convex. The adjacent sur- 

 faces of the crowns and flint being identical they are cemented, and externally 

 the combination is a deep concavo-convex, the concave side facing the view or 

 landscape Its aperture is limited by a diaphragm plate placed at a distance 

 of | the focal length in front of the lens ; the stops provide apertures from 



to . Correction of chromatic and spherical aberration is obtained by the 



12 40 



foci and forms of the lenses employed. 



The angle of picture included exceeds of 70, and though marginal dis- 



