Mr. Cundell on the practice of the Calotype Process. 325 



short distances, however, on account of the increasing diver- 

 gency of the rays, only a small opening, admitting the mere 

 centres of the pencils, can be used with advantage. The size of 

 the plate glasses may be eight inches by six. 



11. It must be observed of this camera, and of all others 

 which are not achromatic, that there is a peculiar adjustment 

 required of the focus, the not attending to which has been the 

 cause of much failure and disappointment. The instrument 

 must be adjusted to what has been appropriately called the 

 chemical focus, which differs materially from the optical or 

 visible focus, as will be seen by the following Table, in which 

 the two are contrasted ; the former being about one thirty- 

 sixth part shorter than the latter for parallel rays, and for di- 

 verging rays in proportion. 



12. It will be found convenient to insert one or more strips 

 of white wood in the sliding part of the camera, as shown in the 

 drawing, and to graduate these with the foci produced by the 

 different "stops" used at E F. This graduation is best done 

 by first accurately determining the visible foci (by daylight) of 

 two fiducial points near the extremities of the scale, by means 

 of a test object and a magnifier, and then setting off by mea- 

 sure the calculated differences ; thus, for a twelve-inch lens, 



* For lenses of a different focus, the graduation will be in proportion, at 

 proportionate distances; thus, for a lens of six inches, the spaces will be 

 one-half the above, at half the distances. 



