the Chemical Rays and those of Radiant Heat. 205 



ing whitened by mercury. Step by step this process goes on, 

 an increased whiteness resulting from the prolonged action 

 or increased brilliancy of the light, until a certain point is 

 gained, and now the iodide of silver apparently undergoes 

 no further visible change; but another point being gained, it 

 begins to assume, when mercurialized, a pale blue tint, be- 

 coming deeper and deeper, until it at last assumes the bril- 

 liant blue of a watch-spring. This incipient blueness goes 

 under the technical name of solarization. 



(50.) The successful practice of the art of Daguerreo- 

 typing, therefore, depends on limiting the action of the sun- 

 ray to the first moments of change in the iodide; for if the 

 exposure be continued too long, the high lights become sta- 

 tionary, whilst the shadows increase unduly in whiteness, and 

 all this happens long before solarization sets in. 



(51.) Let us examine this important phenomenon more 

 minutely. Having carefully cleaned and iodized a silver plate 

 three inches by four in size, it is to be kept in the dark an 

 hour or two. 



By a suitable set of tin-foil screens, rectangular portions of 

 its surface, half an inch by one-eighth, are to be exposed at a 

 constant distance to the rays of an Argand gas-burner (the 

 one I have used is a common twelve-holed burner), the first 

 portion being exposed fifteen seconds, the second thirty se- 

 conds, the third forty-five seconds, the fourth sixty seconds, 

 &c. &c. 



We have thus a series of discs or spaces upon the plate, 

 «, Z», c, d, fig. 3, each of which has been affected by known 

 quantities of light; b being affected twice as much as cr, having 

 received a double quantity of light; c thrice as much as a, 

 having received a triple quantity, &c. &c. 



The plate now is exposed to the vapour of mercury at 

 170° Fahr. for ten minutes ; the spaces or discs all come out 

 in their proper order, and nothing remains but to remove the 

 iodine. 



(52.) An examination of one of these plates thus prepared, 

 shows us* that, commencing with the first space a, we dis- 

 cover a gradual increase of whitening effect until we reach 

 the seventh ; that a perfect whiteness is there attained ; that, 

 passing on to the sixteenth, no increase of whitening is to be 

 perceived, although the quantities of light that have been in- 



* It is impossible to represent these changes in a drawing, which is 

 simply black and white ; it will be understood that the characteristic di- 

 stinction of the spaces from the sixteenth to the twentieth, for example, 

 depends on their assuming a blue tint, which continually deepens in inten- 

 sity. 



