cardboard, cut in the form of the ordinary diaphragm, in place of which it is used. In working iso- or ortho-chromatic plates extreme caution should be exercised as to the illumination of the dark-room ; as little light as possible, and that of only a deep ruby, should be used, and the plate should be covered as far as possible during development. No special developers are required as the plates will work with any ordinary developer, whether pyro, quinol, or ferrous oxalate. Their sensitiveness to white light is generally about the same as other plates, and when used with a yellow screen require from three to ten times the ordinary exposures, according to the depth of tint. The value of colour-sensitive plates is seen in the better rendering of foliage, distance clouds, and water, and in portraiture in the suppression to a great extent of freckles, and the truer rendering of light or golden hair. For copying pictures or coloured objects of any kind these plates are now universally used, and are also of great benefit in photomicrography. The following are the formulae for the princi-pal baths for sensitising ready-prepared plates : -
Solution of erythrosin (i in 1,000) ... I to 2 parts.
Ammonia (10 per cent.)........, 1 part.
Water ............... 8 parts.
Dust the plate, and immerse for two or three minutes. Allow to drain on blotting paper, and dry in the dark. The conclusions drawn from a series of experiments, and given by Mr. Bothamley at the Photographic Convention, 1889, were : "(1) Alcohol up to 10 per cent, has no influence whatever, and may be dispensed with where the dye is soluble in water. Alcohol in larger pro-portion produces a distinct decrease in sensitiveness. (2) With a concentration of the dye up to 1 in St000' tne washing after immersion is totally unnecessary. (3) A preliminary bath may be omitted-"
Azaline solution (see p. 62) ... ... 1 oz.
Distilled water ... ......... ozs.
Alcohol or methylated spirit ...... 4 drms.
Liquid ammonia ......... ... 1 drm.
Dust the plates, and immerse for one minute; rock during immersion. Drain on blotting paper, and dry in the dark.