exposure (it is impossible to express this by rule, as it depends entirely upon the amount of mist in the air)." Distance, then, is scarcely a factor in ordinary landscape or ordinary work, but in enlargement or reduction it becomes a large factor, and may be allowed for by Mr. Debenham's table of relative exposures (p. 305) for varying proportions of image to the original.
Hurter and Driffield s Actinograph is a slide and roller calcu-lating machine which Messrs. Marion supply fitted in a light mahogany box,and it weighs about
4 ozs. Inside and at the back is a cylinder, upon which is a chart showing geographically the intensity of daylight for every hour of each day of the year. The slide next to this cylinder is furnished with two scales - one marked for lens apertures, and the other set out for exposures. Next to this is a small pointer-slide which is adjusted to a fixed plate-speed scale, and the pointer-slide now indicates the exposure for each of six selected and typical meteorological conditions. The instrument sold for this country is plotted out for a latitude of 52o 30'; but instru-ments are supplied for any desired latitude.
Green and Fuidge's Actinometer. This presents the appear-ance of a small box about the size of a watch, on the cover of which are painted seven tints, each successive tint increasing in depth of colour, the same being intended to represent the colour from its lightest to its deepest shade as produced by printing on sensitive albumenised paper. The instrument assumes that the colour obtained by printing on albumenised paper is always the same ; that is to say, that all albumenised papers will print to the same tint, an assumption by no means borne out by experience.
Reid's Actinometer somewhat resembles in appearance a pocket comb, and depends upon the same principle as Green and Fuidge's actinometer, and is open to the same objections.
Watfs Bijou Actinometer. Based on exactly the same principle as the two previous ones.
Stanley's Actinometer. This consists of a reel of paper, coated with a gelatino-bromide emulsion which has been treated with a 10 per cent, solution of nitrite of potassium, which causes a very definite and distinctive coloration by the action of light.
Watkins' Exposure Meters. An old form consists of a brass cylinder with movable pointers and scales. At one end is an actinometer of bromide of silver paper, which is exposed to the