and Laboratory Methods. 



1491 



trum were the only feature to be considered in the selection of a plate for photo- 

 micrographic work, a plate from Group V would be chosen, but the general 

 working of the plate as well as the keeping qualities are factors that must be 

 considered. For practical work and keeping qualities the author found the 

 Cadett & Neal Special Slow Spectrum Plate of Group IV most satisfactory, and 

 used it in the preparation of the spectrographs illustrated in Fig. 3. 



Electric arc. - - 

 Picric acid. - - 

 Aurantia. - - - 

 Cresoidin. - - - 

 Congo Red. - - 

 Eosin. - - - - 

 Carbol Fuchsin. 

 H<Eniatoxylin. - 

 INIethylen Blue. - 

 Green Glass. - - 

 Methyl Green. - 

 Quinine Sulphate. 



Methylen Blue. 



Methyl Green. 

 Picric acid. 

 Picric acid. 

 Methylen Blue. 

 Carbol Fuchsin. 



Fig. 3. 



Explanation to Plate. — This plate is a reproduction of spectrographic analyses of some of the common 

 stains. The red end of the spectrum is on the left. The principal Fraunhofer's lines are marked. 

 The name of the stain is on the left, while on the right is the name of the proper screen to use to 

 increase the photographic intensity. To decrease the contrast use a screen of the same color as the 

 stain. 



The perfect photo-micrographic plate would give equal photographic in- 

 tensity to all the colors of the visual spectrum, but since this degree of excellence 

 has not been attained, the value of a given plate may be enhanced by the use of 

 color screens, or ray filters, which serve to increase or decrease the photographic 

 intensity of a color. The following are the laws upon which the use of color 

 screens is based : 



First. To increase the photographic intensity of a color, a screen of 

 complementary color should be used. 



Second. To decrease the photographic intensity of a color, a screen of the 

 same color should be used. 



These laws hold good for all branches of photography, whether by trans- 

 mitted or reflected light, and are not dependent upon the position of the screen, 

 i. e,, whether the screen is placed between the source of illumination and the 

 object, between the object and the objective lens, or between the lens and the 

 plate. 



