COLOR PHOTOGRAPHY 631 



obtained at once by taking the ratio of the standard reference transmission above 

 (5 per cent) to 2.52. This gives 5/2.52 = 2 approximately. The blue-green bromide 

 for this new set should therefore be exposed 12 X 2 = 24 sec, approximately, at//8 

 and 3X magnification. Its correct exposure for different values of lens stop and 

 different magnifications can be derived quickly from the enlargement exposure 

 calculator. 1 



The saving of time and material and the uniformity of results which can be realized 

 by a systematic procedure must not be underestimated. 



Carbro Solutions. — The carbro solutions for sensitizing the carbon tissues consist 

 of a bleaching agent as potassium ferricyanide with potassium bromide as accelerator, 

 and hardening agents as potassium dichromate with chromic acid for controlling the 

 contrast. 



Alternative solutions consist of potassium ferricyanide, potassium bromide, and 

 potassium dichromate as bleaching and hardening agents and glacial acetic acid and 

 hydrochloric acid as controlling agents. 



Potassium chrome alum or formaldehyde are added in small quantities to the above 

 solutions for the purpose of producing a slight superficial tanning in order to preserve 

 the high lights. 



Typical Stock Solutions 



(Type I Solutions Using Chromic Acid) 

 Stock Solution .4. 



Potassium ferricyanide 1 oz. 50 g. 



Potassium bromide 1 oz. 50 g. 



Water (distilled) to 10 oz. 500 cc. 



Stock Solution B 



Potassium dichromate (c.p.) 180 gr. 18 g. 



Chromic acid 40 gr. 4 g. 



Chrome alum 100 gr. 10 g. 



Water (distilled) to 10 oz. 450 cc. 



WoBKiNG Bath 1 



Stock solution A 1 part 



Water 4 parts 



Working Bath 2 



Stock solution B 1 part 



Water 4 parts 



Each color tissue is immersed in working bath 1 for 3 min., drained for 10 or 15 sec. 

 or squeegeed gently face down on a clean piece of plate glass to remove the surplus 

 of bleaching solution, and then immersed in working bath 2 for a time which might 

 vary from 15 to 40 sec. depending on the type of bromide paper used and on the degree 

 of contrast desired. 



After removal from bath 2, each tissue is squeegeed on its appropriate bromide and 

 left in contact with it for 10 or 15 min. 



The precautions to be followed in sensitizing and the method of squeegeeing, etc., 

 will be considered later. 



With Type II solutions also each color tissue is immersed in working bath 1 for 

 3 min,, drained for 10 or 15 sec, and then immersed in working bath 2 for a time which 

 might vary from 15 to 40 sec. depending on the type of bromide and degree of contrast 

 required. 



It has been often recommended, particularly with tissues which had too great a 

 tendency to frill, to reduce the time of immersion in the No. 1 bath down to 2 min., 



1 A photoelectric exposure meter with a full scale sensitivity of 7.5 ft. -candles can be obtained from 

 the General Electric Co. on special order and at a price slightly higher than the standard model. The 

 shape and sensitivity of this meter are such as to permit any kind of measurements of the light from 

 enlargers. Its indications are, of course, equally correct with diffused- and condenser-type light sources. 



