174 The Microscope. 



better than those of the wet-plate process. As these plates are 

 sensitive to both orange and ruby light, the dark room must be 

 especially prepared for handling them; for this purpose the ordinary 

 ruby glass of the dark room should be covered with one thickness of 

 orange paper, and then by one of brown tissue paper; even in this 

 light the plates should not be exposed for any length of time. The 

 length of exposure for the orthochromatic plates varies, of course, 

 with the amount of light and the objective used; roughly speaking, 

 it may be said to range from one second to fifteen or twenty seconds. 

 We have exposed a plate one second with a Tolles 4-inch objective, 

 and found it not over-exposed, and with a Powell & Leland 1-25 

 bom. im. objective we have given as high a"s fifteen seconds exposure 

 with perfect results. A little practice will soon enable the operator 

 to determine the necessary amount of exposure for a certain quantity 

 of light. It will be found best to make the mistake of a slight over- 

 exposui'e than that of under-exposure, as the former is more easily 

 corrected in the process of development by restraining. 



Of the different developers, ferrous oxalate, pyro- and hydro- 

 chinon, the latter is the only one that will give uniformly perfect 

 results in photo-micrography. Of three plates developed respectively 

 in ferrous oxalate, pyro- and hydrochinon and exposed for the same 

 length of time at the same object, that developed by the hydrochinon 

 was the only one found to be perfect and at aU approaching a wet 

 plate in printing quality. Owing to the slowness of this developer 

 a quality of great advantage for photo-micrography and lantern 

 slides, the operator I'uns less risk of spoiling a good negative by 

 over- development. The hydrochinon developer may be used over 

 and over again, and this fact, viewed from an economical standpoint, 

 is well worth considering. 



The following is a formula for the hydrochinon developer to be 

 highly recommended for photo-micrography and lantern slides: 



No. 1. 



* I^ Soda Carbonate 400 grains. 



Water 8 oz. 



No. 2. 



Hydrokinone 96 grains. 



Soda Sulphate , 480 " 



"Water 8 oz. 



Developer No. 1, two drachms; No. 2, four drachms; water, two 

 ounces. Add one or two drops of a ten per cent, solution of bromide 

 of potassium. 



* The Philadelphia Photographer, December 17, 1887. 



