MEMOIR XVII.] CHEMICAL RAYS AND RADIANT HEAT. 237 



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

 cover a gradual increase of whitening effect until we 

 reach the seventh ; that a perfect whiteness is there at- 

 tained ; 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 con- 

 tinually 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 increments of incident li^ht. 



O 



Or, by presenting a plate covered with a screen to a 

 sky that is clear or uniformly obscured, and with a regu- 

 lar 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 plate, as is 

 represented in Fig. 37; 

 from a to b the changes 

 have been successive; from 

 f) to c no variation in the 

 amount of whitening is per- 

 ceptible ; at d solarization 



is commencing, which becomes deeper and deeper to the 

 end, , of the stain. 



* 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 Hue 

 tint, which continually deepens in intensity. 



