First steps in photo-micrography / [F. Martin Duncan].

  • F. Martin Duncan
Date:
1902
Catalogue details

Licence: In copyright

Credit: First steps in photo-micrography / [F. Martin Duncan]. Source: Wellcome Collection.

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    WATSON'S NEW CONVERTIBLE LENS, THE HOLOSTIGIYIAT. Series I. F/6*I. A rapid universal lens, unrivalled for groups, portraits, architecture, landscape process, and all work requiring great rapidity. This lens is of the convertible type, and by the use of the back or front combinations alone, a choice of two extra focal lengths is ob- tained. Angle of view with doublet, 65°. With single com- binations, 36° and 430 respectively. PRICE LIST. Single Lenses Aperture f/n’s- Combined Focus. Working Aperture. Size of Plate. Prices, with Iris Diaphragm. .Focus of Front. Focus of Back. ins. ins. ins. ins. £ s- d. 7} 6 4 f/6-S 4x3 4 10 0 ?i 7* 4t i/6-r 4r x 35 5 0 0 84 7} 42 f/6'5 4^ X 47,- 5 10 0 81 8* 5 i/6 ‘ r 5x4 6 0 0 icj 84 54 f/6‘5 54 x 4 j 6 7 6 ioi iof 6 f/6'i 6x5 6 15 0 12J ioi 6-4 i/6.5 61 x 4j 7 5 0 12J I2i 7 f/6-i 7x5 7 15 0 143 I2i 7$ 176-5 8x6 8 12 6 14* i4i 84 i/n 8jx 64 9 10 0 I7i 144 9i 1/6-5 9x7 11 15 0 *7* 17} IO i/6 • 1 10 x 8 14 0 0 20ir 17} II 1/6-5 ii x 9 16 0 0 2oi 2o4 12 f/6'1 10 X 10 18 0 0 Subject to a Cash Discount of 5 per cent. Tlie following extracts from a criticism of one of these Lenses (of 63 in. combined focus) which appeared in Photography in July last will be of interest:— “ The largest effective aperture possible was one inch in diameter, so it will be seen that the lens was capable of working at f/6-4. With this stop it could be got to cover a whole-plate easily with good definition, and on a ten by eight plate, only the extreme corners were unillu- minated, the angle of the cone of rays being 86°. It will be observed that the actual angle required by a half plate lens of this focus is 64°, so that there was an excellent margin for the use of a rising front, whether the plate were used vertically or horizontally.” The tests applied for achromatism showed conclusively that the correction in this respect left nothing to be desired, while in the matter of distortion, the image of a straight line the full length of a half-plate, faUing right upon the margin of the plate, did not vary from the straight more than a hundredth of an inch in its whole course—an amount quite imperceptible even to a practised eye without instrumental aid. The curvature of the field was simUarly infinitesimal Tested on a flat subject, ON A WHOLE PLATE, the extreme edges were at their sharpest focus, with full aperture, only one-fiftieth of an inch nearer to the lens than the centre. This was the mean of a series of readings, which did not vary amongst themselves a hundredth of an inch altogether, and should suffice to show that the lens is not merely a medium angle high rapidity lens, but should prove a very valuable wide-angle instrument also.” W. WATSON & SONS, 313, HICH HOLBORN, LONDON, W.C., and 16, FORREST ROAD, EDINBURCH. (Opticians to H.M. Government.)
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