278 PKACTICAL PHOTO-MICEOGBAPHY 



as otherwise, in case one negative were over-exposed, some 

 spreading action of the light might occur, and its image would 

 tend to be somewhat larger than either of the others. 



The most suitable illuminants are those which approximate 

 most nearly to daylight. A source of light, therefore such as 

 a mercury- vapour lamp is not available since its spectroscopic 

 value differs so materially from any other. The most suitable 

 illuminants are acetylene, the Nernst electric lamp, or lime- 

 light. The arc lamp, perhaps, is not generally so satisfactory, 

 owing to variability in the composition of the carbons, and to 

 the possibility of change in the actinic value of the light with 

 varying voltages and current density, which would disturb the 

 balance so necessary to be maintained between the three 

 negatives. 



The negatives are obtained successively through the 

 coloured niters mentioned, and the three resulting negatives 

 are developed in the same solution for the same time so as to 

 obtain as nearly as possible the same type of negative in each 

 case. It need hardly be pointed out that, as the plate used 

 has to be sensitive to practically the whole spectrum, the dark- 

 room illumination must be such that plates of this sensitiveness 

 may be safely dealt with. 



Up to this point the production of the negatives the 

 process is one that differs in no respect from ordinary photo- 

 graphy. From each of these negatives a positive has to be 

 obtained. From the negative taken through the red screen 

 an ordinary lantern-slide on glass is made, and this is then 

 toned to a suitable blue colour with the solution supplied for 

 the purpose by the makers. From the remaining two negatives 

 a positive in bichromated gelatin has to be made. The films, 

 which are coated on celluloid, may be procured ready sensitized 

 from the makers, and should be dealt with exactly in accordance 

 with their instructions. They are printed from the negative 

 inexactly the same way as in obtaining an ordinary lantern-slide, 

 the sensitized surface being placed in contact with the film-side 

 of the negative in an ordinary printing-frame and exposed to a 

 suitable source of light. The exposure must be determined by 

 an actinometer, as it is most important to see that this is correct ; 

 but in any case some preliminary experiments may be necessary 

 before the exact duration is arrived at. On removal from the 



