152 THE CHEMICAL RAYS BECOME LATENT. 



612. But, in truth, it is not so. When the rays begin to act on it, the iodide com- 

 mences changing, and is capable of being 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 ap- 

 parently undergoes no farther visible change ; but another point being gained, it begins 

 to assume, when mercurialized, a pale-blue tint, becoming deeper and deeper, until it 

 at last assumes the brilliant blue of a watch-spring. This incipient blueness goes un- 

 der the technical name of solarization. 



613. The successful practice of the art of Daguerreotyping, 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 stationary, while the shad- 

 ows increase unduly in whiteness, and all this happens long before solarization sets in. 



614. Let us examine these important phenomena 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. 



615. 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 seconds, the third forty-five seconds, 

 the fourth sixty seconds, &c., &c. 



616. We have thus a series of discs or spaces upon the plate (a, b, c, d,fig. 96), each 

 of which has been affected by known quantities of light; b being affected twice as 

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

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



617. The plate now is exposed to the vapour of mercury at 170 Fah. for ten min- 

 utes ; the spaces or discs all come out in their proper order, and nothing remains but 

 to remove the iodine. 



618. An examination of one of these plates thus prepared shows us* that, commen 

 cing with the first space a, we discover 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 incident and absorbed have been continually increasing ; but as soon 

 as the light thus latent has reached a certain quantity, visible decomposition sets in, 

 indicated by a blueness, and the sensitive surface once more renders evident the incre- 

 ments of incident light. 



619. Or, by presenting a plate covered with a screen to a sky that is clear or uni- 

 formly obscured, and with a regular motion, withdrawing the screen deliberately from 

 one end to the other, and then suddenly screening the whole, it is plain that those 

 parts first uncovered will have received the greatest quantity of light, and the others 

 less and less. On mercurializing, it will be seen that a stain will be evolved on the 



* It is impossible to represent these changes in a drawing, which is simply black and white ; it will be understood that 

 the characteristic distinction of the spaces, from the sixteenth to the twentieth, for example, depends on their assuming a 

 blue tint, which continually deepens in intensity. 



