THE PRODUCTION OF POSITIVES, ETC. 145 



of the objective. As £ when the aperture is half 

 an inch and the focal length of the objective ten 

 inches and so on, the smallest aperture generally 

 employed being one fiftieth of the focal length or J^. 

 A combination of great focal length will bear a 

 smaller aperture proportionally than a shorter one. 

 For instance, with a negative possessing the char- 

 acteristics before mentioned, a quarter plate photo- 

 graphic objective requires an aperture = J z while 

 one = Jg will be necessary when using a half plate 

 portrait combination. 



We now proceed to the dark room, and having cut 

 our gelatino-bromide paper to the required size, float 

 the unprepared side on the surface of some perfectly 

 clean water, (the greatest care must be taken to 

 prevent the water from flowing over the prepared 

 side). When perfectly flat it is laid evenly on a 

 sheet of patent plate and finally placed in the dark 

 slide. Some recommend the complete immersion 

 of the paper in water before laying it on the glass 

 plate, but the length of time necessary for perfect 

 drainage, and the risk of spoiling the picture if 

 sufficient time is not allowed for it, has led us to 

 adopt the former plan, which, if it requires more skill 

 and care, has at least the advantage of celerity. 



After an exposure of say ten seconds, we return 

 to the dark room and on removal of the paper from 

 the dark slide, replace it in the water. A sufficient 

 quantity of ferrous oxalate developer, in the propor- 

 tion of five parts of a saturated solution of oxalate of 

 potash to one of ferrous sulphate (care being taken 



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