236 



CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



The successful practice of the art of daguerreotyping, 

 therefore, depends oil 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 shadows increase unduly in white- 

 ness, and all this happens long before solarization sets in. 

 Let us examine this important phenomenon more mi- 

 nutely. 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 por- 

 tions 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 sec- 

 onds, the second thirty seconds, the third forty-five sec- 

 onds, the fourth sixty seconds, etc. 



We have thus a series of 

 spaces upon the plate, , 5, 

 c, d, Fig. 36, each of which 

 has been affected by known 

 quantities of light ; b being 

 affected twice as much as , 

 having received a double 

 quantity of light; c thrice 

 as much as , having re- 

 ceived a triple quantity, 

 etc. 



The plate is now exposed 

 to the vapor of mercury at 

 170 Fahr. for ten minutes; 

 the spaces all come out 

 in their proper order, and 

 nothing remains but to remove the iodide. 



An examination of one of these plates thus prepared 



Fig. 36. 



