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



tecting the picture from all external light, except that which 

 emanates from the objects to be copied ; the rays from the 



direction b being intercepted at B, and those from d at D. 

 The paper is placed between two plates of glass, introduced 

 at the open end G H, and these are pressed together and se- 

 cured in their place by means of a detached door having a re- 

 volving bar behind it, the extremities of which work in grooves 

 in the sides of the outer case *. 



9. By reference to the diagram, it will be seen that by means 

 of the diaphragm or " stop " E F, the rays from the barb of 

 the arrow are excluded from the upper and received only upon 

 the lower half of the lens, upon which they fall at a compara- 

 tively high and equal angle of incidence. They are thus less re- 

 fracted than they would otherwise be, and their focus is not only 

 sharpened but elongated. By this means, the picture, instead 

 of being formed in the usual curve, is formed much nearer to 

 a straight line in the plane of the paper placed to receive it. 



10. A lens of twelve inches focus ought to have an aperture 

 of 2'4 inches. The diaphragm at E F (in which the princi- 

 pal virtue of the instrument resides) ought to be placed 1 '5 

 inch in advance of the lens, and its opening ought not to ex- 

 ceed 1*2 inch. By using one of a smaller opening, a much 

 finer image will be obtained, but at the sacrifice of light : at 



* The instrument, much improved, may be had of Mr. Dennis, 118 Bi- 

 shopsgate Street Within. 



