214 PEACTICAL PHOTO-MICKOGKAPHY 



spectroscope to ascertain whether the screen in use is absorbing 

 the particular portion that is not required. 



When using a mercury- vapour lamp the conditions are some- 

 what different to those occurring with any other illuminant. 

 As already stated, the spectrum of mercury vapour is confined 

 principally to three bright lines : one in the yellow or more 

 correctly, a double line in the yellow a brilliant green line, 

 and one in the blue (Fig. 67 I). There is also a violet line 

 which is visually not very evident although photographically 

 very active. It is therefore only necessary to screen off the 

 lines not required, when the remaining one becomes a truly 

 monochromatic source of light. The dyes that will be re- 

 quired to make a series of niters for this light are : 



Filter-yellow K ; 

 Tartrazine ; 

 Acid-green ; 

 Methyl- violet, and 

 Eosin, soluble in water. 



To transmit only the double yellow line, two solutions should 

 be made : filter-yellow K and eosin, each of a strength of one 

 in one thousand, for use in a cell five millimetres thick. To 

 transmit only the green line a solution of acid-green, tartrazine, 

 and filter-yellow K should be used. In this case the three dyes 

 may be made up into one solution, or rather separate solutions 

 made, and then equal quantities taken of each. This, as a 

 general rule, is an unsafe procedure with aniline dyes, since they 

 are very liable to interact. The absorption-spectrum of the 

 solutions becomes altered in many cases on mixing, or a 

 precipitate may be thrown down. 



To transmit only the blue and violet bands a solution of 

 methyl-violet, one in five thousand, is sufficient. Owing to 

 the ease and certainty with which monochromatic illumination 

 can be obtained by means of the mercury -vapour lamp, it is 

 for difficult objects, or faintly stained preparations, of the 

 greatest possible service. 



Another method of obtaining monochromatic light is a 

 spectroscopic method in which the light is split up, either by 

 a prism or by a diffraction-grating mounted on a prism, into 

 a continuous spectrum. A convenient apparatus for doing 



