322 Mr. Cundell on the practice of the Calotype Process. 



improved upon ; and it is with the hope of promoting its im- 

 provement, by removing some of the difficulties left at the 

 threshold, and opening the way for the entrance of labourers 

 into the vineyard, that I have been tempted to offer this little 

 treatise to the public. They had been better pleased, no doubt, 

 to have received such an offering from the hands whence it 

 ought to have come; but with every respect for the distin- 

 guished author of the calotype, I hope I may without impro- 

 priety do that which he has omitted to do, by furnishing plain 

 directions, from my own experience, by which calotype pic- 

 tures may be produced, without much difficulty and with tole- 

 rable certainty and success. 



4>. The Daguerreotype plate owes its sensibility to the 

 iodide of silver, obtained by exposing the metal to the vapour 

 of iodine. The same compound, iodide of silver, is the foun- 

 dation of the calotype also; but it is obtained by a "humid" 

 process, by the decomposition of nitrate of silver, upon the 

 surface of paper, by means of a solution of the iodide of po- 

 tassium. It has been found that paper so prepared, when 

 treated with gallic acid, becomes exceedingly sensitive; and 

 that upon the slightest exposure to daylight, under particular 

 treatment, it will become perfectly black and opaque. Hence 

 its fitness and adaptation to receive the delicate but feeble im- 

 pressions of the images formed in the camera obscura, which 

 imprint upon it what has been called a "negative" picture, 

 having the lights and shadows of nature reversed. This "ne- 

 gative," when fixed and rendered permanent, is used as a 

 matrix; and, by a simple and well-known process, a great num- 

 ber of impressions may be photographically printed from it, 

 representing objects not only in true light and shadow, but 

 true also in relation to right and left. 



5. Before anything good can be produced in calotype, the 

 operator must be provided with a properly constructed camera 

 obscura. The cameras met with in the shops are generally 

 made after the French model, with nominally achromatic lenses, 

 of the plano-convex figure, and of a short focus. Without 

 presuming to disparage these, which no doubt will give a por- 

 tion of well-defined picture in the centre of the field, sufficient 

 for a single portrait, I would venture to recommend, on the 

 authority of Dr. Wollaston, a lens of the meniscus figure, 

 having the radii of its curves in the proportion of two to one. 



6. He has shown, in an essay on the particular subject (in 

 the Philosophical Transactions for 1812), that the meniscus 

 figure, when properly "stopped"* is peculiarly adapted to the 



* His improvement is a very striking one ; and it seems odd, that the 

 principal part of it, upon which the effect chiefly depends, his mode of stop- 



