THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY, 293 



quantity of salt ami that of the solution of silver which answers best and 

 gives the niaxiniuiu elTect. If the strenjith of the salt is augmented beyond 

 this jioiiit the effect diminishes, and, in certain cases, becomes exceedingly 

 small. 



This pajier, if properly made, is very useful for all photographic puri)Oses. 

 For example, nothing can be more perfect than the images it gives of leaves 

 and flowers, especially with a summer sun, the light passing through the leaves 

 delineates every ramiflcation of their nerves. 



Now, suppose we take a sheet thus prepared and \\'ash it with a saturated 

 solution of salt and then dry it. We shall find (especially if the paper is kept 

 some weeks before trial is made) that its sensibility is greatly diminished, and 

 in some cases seems quite extinct. But if it is again washed with a liberal 

 quantity of the solution of silver, it becomes again sensible to light, and even 

 more so than it was at first. In this way, by alternately washing the paper 

 with salt and silver, and drying it l)etween times, I have succeeded in increas- 

 ing its sensibility to the de.gree that is requsite for receiving the images of the 

 camera obscura. 



In conducting this operation it will be found that the results are sometimes 

 more and sometimes less satisfactory in consequence of small and accidental 

 variations in the proportions employed. It happens sometimes that the chloride 

 of silver is disposed to draken of itself without any exposure to light. This 

 shows that the attempt to give it sensibility has been carried too far. The object 

 is to approach to this condition as near as possible without reaching it, so that 

 the substance may be in a state ready to yield to the slightest extraneous force, 

 such as the feeble impact of the violet rays when nuich attenuated. Having, 

 therefore, prepared a number of sheets of paper with chemical proportions 

 ^■lightly ditferent from one another, let a piece be cut from each, and, having 

 been duly marked or numliered, let them be placed side by side in a very weak 

 diffused light for a quarter of an hour. Then, if any one of them, as frequently 

 happens, exhibits a marked advantage over its competitors, I select the paper 

 which bears the corresponding number to be placed in the camera obscura. 



The increased sensitiveness given to paper by alternate ablutions 

 of saline and argentine washes, the striking differences of effect pro- 

 duced by accidental variations of the proportions in which the chem- 

 ical ingredients are apj)lied, and the spontaneous change which takes 

 place, even in the dark, on the more sensitive varieties of the paper, 

 are all subjects of great interest, which demand further investiga- 

 tion than they have ever j^et received, and which, if followed out, 

 promise some most important explanations of chemcial phenomena 

 at present involved in uncertainty, particidarly those which appear 

 to show the influence of time, an element not sufliciently taken into 

 account, in overcoming the weaker affinities. Few fields of research 

 promise a greater measure of reward than these. Already the art 

 of making sun pictures has led to numy very important physical dis- 

 coveries, but most of the phenomena are yet involved in obscurity. 



C. Daguerreotype — The Drsco\'ERV ov M. Daguerre. 



. It has already been stated that Niepce and Daguerre, having by 

 accident discovered they were prosecuting experiments of the same 

 kind, entered into partnership. On the 5th of December, 1829, 



