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blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of 18° When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity. Another plan, based on the same principles, is to place segments of annul uses of vermilion, chrome yellow, emerald green, French blue and white on a disk, and to complete the annul uses with black segments, the amount of black depending on the luminosity of the pigments, which can be readily measured. When the disk is rotated, rings of colour, modified in brightness by black, are seen, and each ring will be of the same luminosity. As before, a screen (light-filter) to be used in front of the lens must be found which will cause the developed images of all the rings to appear of equal opacity. It must be remembered that the light in which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured.

Action of the Spectrum on Chromic Salts.—The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of the various parts of the spectrum on potassium bichromate.

Fig. 2—The top letters have reference to the Fraunhofer lines; the bottom letters are the initials
of the colours. The relative sensitiveness is shown by the height of the curve above the base-line.

If other bichromates are employed, the action will be found to be tolerably well represented by the figures. No. 1 is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen.

Action of the Spectrum on Asphaltum.—This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents.

Action of the Spectrum on Salts of Iron.—The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced. We give this as a representation (fig. 3) of

Fig. 3.—Same description as for fig. 2.

the spectra obtained on ferric salts in general. Here, again, we have an example of the law that exists as to the correlation between absorption and chemical action. One of the most remarkable compounds of iron is that experimented upon by Sir I. Herschel and later by Lord Rayleigh, viz ferrocyanide of potassium and ferric chloride. If these two be brushed over paper, and the paper be then exposed to a bright solar spectrum, action is exhibited into the infra-red region. This is one of the few instances in which these light-waves of low re frangibility are capable of producing any effect. The colour of this solution is a muddy green, and analysis shows that it cuts off these rays as well as generally absorbs those of higher re frangibility.

Action of Light on Uranium.—The salts of uranium are affected by light in the presence of organic matter, and they too are only acted upon by those rays which they absorb. Thus nitrate of uranium, which shows, too, absorption-bands in the green blue, is affected more where these-occur than in any other portion of the spectrum.

Some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are affected by light, but in a less degree than those which we have discussed. In the organic world there are very few substances which do not change by the continuous action of light, and it will be found that as a rule they are affected by the blue end of the spectrum rather than by the red end (see Photochemistry)

The following table gives the names of the observers of the action of light on different substances, with the date of publication of the several observations. It is nearly identical with one given by Dr Eder in his Geschichte der Photo-Chemie.

Substance.		Observer.	Date.
Silver
Nitrate solution mixed with chalk, gives in sunshine copies  of writing		J. H. Schulze	1727
Nitrate solution on paper		Hellot	1737
Nitrate photographically used		Wedgwood and Davy.	1802
Nitrate on silk	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Fulhame	1797
		Rumford	1798
Nitrate with white of egg		B. Fischer	1812
Nitrate with lead salts		Herschel	1839
Chloride		J. B. Beccarius	1757
Chloride in the spectrum		Scheele	1777
Chloride photographically used		Wedgwood	1802
Chloride lackened		Lassaigne	1839
Iodide		Davy	1814
Iodide by action of iodine (on metallic silver)		Daguerre	1839
Iodide photographically used		Herschel	1840
Iodide with gallic acid		Talbot	1841
Iodide with ferrous sulphate		Hunt	1844
Chloride and iodide by chlorine and iodine (on metallic silver)		Claudet	1840
Bromide		Balard	1826
Bromide by action of bromine (on metallic silver).		Goddard	1840
Sulpho-cyanide		Grotthus	1818
Nitrite		Hess	1828
Oxide with ammonia		Mitscherlich	1827
Sulphate		Bergmann	1779
Chromate		Vauquelin	1798
Carbonate		Buchholz	1800
Oxalate		Bergmann	1779
Benzoate		Trommsdorf	1793
Citrate		Vauquelin	1798
Kinate		Henry and Plisson	1829
Borate		Rose	1830
Pyrophosphate		Stromeyer	1830
Lactate		Pelouze and Gay-Lussac	1833
Formiates		Hunt	1844
Fulminates		Hunt	1844
Sulphide by vapour of sulphur (on metallic silver)		Niepce	1820
Phosphide by vapour of phosphorus (on metallic silver)		Niepce	1820
Gold.
Oxide		Scheele	1777
Chloride on paper		Hellot	1737
Chloride on silk		Fulhame	1794
Chloride in ethereal solution		Rumford	1793
Chloride with ferrocyanide and ferricyanide of potassium.		Hunt	1844
Chloride and oxalic acid		Dobereiner	1831
Chromate		Hunt	1844
Plate of gold and iodine vapour		Goddard	1842