200 PKACTICAL PHOTO-MICKOGRAPHY 



To make a dark-room lamp of the tank form suitable for 

 work with colour sensitive plates, a solution such as the follow- 

 ing may be used. To 100 c.c. of water add one gramme of 

 aurantia, and 0'25 gramme of methylene-blue. The exact 

 strength of solution should be determined by means of a hand 

 spectroscope, as the light transmitted will, of course, depend 

 on the thickness as well as on the strength of the solution ; 

 so that the strength should be such that it transmits a band of 

 the spectrum limited between 5600 to 5800 X. 



Another dark-room safe light, suitable for a lamp of the 

 liquid-cell type, may be made up with a solution as follows : 



Tartrazine .... 0-5 gramme. 

 Violet dahlia .... 0-2 gramme. 

 Water 1000 c.c. 



The solution should be filtered. This screen is suitable 

 for all plates that are sensitive to yellow and green, and would 

 therefore be suitable for most of those of the orthochromatic 

 or isochromatic type. For very rapid or panchromatic plates, 

 the quantities of the above dyes should be increased, and may 

 in extreme cases be even doubled. It is stated that with a ten 

 candle-power incandescent electric lamp used behind such a 

 screen no fog is caused on panchromatic plates even when 

 development is prolonged. The thickness of the cell should be 

 approximately one inch, but may be less than that where an 

 illuminant of lower intensity is used. 



The ordinary photographic worker who is accustomed to red 

 light in the dark-room at first gets the impression that green light 

 cannot possibly be safe for his plates ; but this is entirely owing 

 to the fact that the eye is much more sensitive to green than 

 it is to red, so that a smaller quantity of green light can be 

 safely used than is possible when using red light. After 

 having once become thoroughly used to this method, it will be 

 found that it is easier for all classes of work, and probably less 

 harmful to the eyes of the worker, than any other source of light. 



The dark-room illumination should never be effected by 

 daylight, as the source of light is too variable, and, further, the 

 effect of daylight on either ruby or orange glass or on the green 

 glass filters, is very marked. In any case, every few months 

 the dark-room light should be tested to ensure that no alteration 



