208 SEC. 7. LIGHT. 



The rati of foci between the front and back combinations are in the propor- 

 tion of 2 3 nearly, and the radii of curvatures are calculated to produce well 



denned images for aperture = ^, and angle of picture = 30. 



959b. Photographic Lenses for the reproduction of maps, 

 plans, &c., as practised at the Home and Foreign Government 

 Topographical Establishments. Joseph Henry Dallmeyer. 



1. A triple achromatic lens of 18" focal length for copying on plates 15 x 12. 

 As its name implies, this lens consists of three achromatic, or actinic- combi- 

 nations, two of which, i.e. the front and back are positive or converging lenses, 

 and between these two is positioned a negative or diverging lens. The 

 positive combinations, the front of 2" and the back of 3" diameter respectively, 

 with focal lengths of similar proportion, are composed each of a double 

 convex crown and a plano-concave flint-glass lens ; the adjacent surfaces are 

 identical and cemented. The convex or crown lenses occupy the external 

 position in both, and the combinations are separated by an interval equal to 

 the diameter of the largest or posterior lens. Between the two, and propor- 

 tionate to their diameters or foci, is placed the diaphragm aperture or stop ; 

 this is also the position of the negative achromatic combination l" diameter, 

 the crown of which is in this case a double concave, and the flint a piano con- 

 vex nearly. This combination limits the aperture to ; it does not affect the 



direction of the pencils as refracted by the front and back positive combina- 

 tions, which produce an image free from distortion, though too much curved 

 to admit of its reception on- a flat screen ; but its action (that of central 

 pencils only) is confined to the proportionately greater prolongation of the 

 oblique or marginal pencils, in virtue of its negative or divergent power, or, 

 in other words, it lengthens these pencils and produces the required amount 

 of flatness of field, the sine qua non for copying purposes. 



This lens, one of a series, was introduced in 1860, and is reported upon by 

 the jurors of the International Exhibition of 1862: "As the first aplanatic 

 " non-distorting view lens placed within the reach of photographers, and the 

 " best lens extant for copying purposes, &c." It was first used in the 

 Ordnance Survey Office at Southampton. 



2. A 3" symmetrical combination of 24" focal length for plates 18" x 16". 

 This lens, constructed for copying, consists of two combinations identical in 

 all respects ; each has a focal length of 46", and is composed of a convexo- 

 concave flint, and a convexo-concave or meniscus crown-glass lens ; the radii 

 of curvatures of the concave of the one and the convex of the other are 

 identical and they are cemented. 



The combinations, with their convex or flint elements external, are 

 mounted in a tube a given distance apart, in this case about ^ of the compound 

 focal length, and midway between them is the perforation" for the insertion of 

 diaphragms or stops. Both combinations being identical in form and focal 

 length the directions of the finally emergent pencils are parallel to the incident 

 ones ; hence the lens produces images free from distortion. 



Flatness of field and correction of aberrations are obtained, for the par- 

 ticular kind of glass employed, by the forms and foci of the component ele- 

 ments, and in order to suit particular requirements the construction is modified 



f f 



for varying angular apertures of from to ^-, including proportionate angles 



of pictures of from 35 to 90. This lens was introduced in 1866. 



96O. Photographic Lenses for Portraits and Views. 



Joseph Henry Dallmeyer, 



