TlIK IlISTOKY OF SOMK DISCOVKKIKS OK PlIOTOGKAI'f I V. 808 



Paper thus j)iv])ar('(l is oxciuisitcly scnsKive to liirlit, an exposure 

 of less than a second to diii'iised daylii>:lit beinfr quite suflieient to set 

 up the process of change. If a piece of tliis paper is partly covered 

 and the other exposed to dayli<rht for tlie briefest possible period of 

 time, a very decided impression ^vill be made.. This impression is 

 latent and invisible. If. however, the j)aper be placed aside in the 

 dark, it will <rradually develop itself; or it may be brought out im- 

 mediately by being washed over with the <;all()-nitrate of silver, and 

 held at a short distance from the fire, by which the exposed portions 

 become brown, the covered ])arts remainin<>: «f their ori^jinal color. 

 The pictures being thus procured, are to be fixed by washing in clean 

 water and lightly drying between blotting paper, after which they 

 are to be washed over with a solution of bromide of potassium con- 

 taining 100 grains of that salt dissolved in 8 or 10 ounces of water; 

 after a minute or two it is again to be dipped into water, and then 

 finally dried. 



Such was, in all its main features, the description given by Mr. 

 Talbot in his specification of his process for producing the calotype, 

 or beautiful jjicture (as the term signifies). He, in a second patent, 

 included the points stated in the next section. 



SECTION III. IMPROVEMENTS IN CALOTYPE. 



Such is the term employed by Mr. Talbot, and these improvements 

 consist of the following particulars, constituting that gentleman's 

 second claim : 



1. Removing the yellowish tint which is occasioned by the iodide 

 of silver from the paper by plunging it into a hot bath of Iwposul- 

 phite of soda dissolved in ten times its weight of water and heated 

 nearly to the boiling point. The picture should remain in the bath 

 about ten minutes and be then washed in warm water and dried. 



Although this has been included by Mr. Talbot in his specifica- 

 tion, he has clearly no claim to it, since in February, 1840, Sir John 

 Herschel published, in his memoir " On the chemical action of the 

 rays of the solar sj^ectrum," a process of fixing with the hot hyposul- 

 phite of soda. 



After undergoing the operation of fixing the picture is placed upon 

 a hot iron and wax melted into the pores of the paper to increase 

 its transparency. 



2. The calotype paper is rendered more sensitive by placing a 

 warm ii'on behind in the camera while the light is acting upon it. 



3. The preparation of io-gallic pajjer, which is simply washing a 

 sheet of iodized paper with gallic acid. In this state it will keep in 

 a portfolio, and is rendered sensitive to light by washing it over 

 with a solution of nitrate of silver. 



4. Iodized paper is washed with a mixture of 26 parts of saturated 



