234 PHOTOGRAPHY WITH COLOR SCREENS [Cn. VII 



to avoid the shadows between the filaments. When a diffuser is 

 used with the mazda or arc light the diaphragm of the objective 

 can be closed as much as desired, but of course it then takes a much 

 longer exposure. If now one uses a 6- volt mazda head-light lamp 

 by inserting a transformer in the circuit for the alternating cur- 

 rent, or by using a storage battery for the direct current, the fila- 

 ment is so concentrated that the source may be treated like that 

 of an arc light, and no diffuser used. This makes it possible to 

 use the full opening of the objective. The candle power of the 

 6 volt mazda is much less than that of the arc light, but it has 

 the advantage of requiring no attention after being once centered. 

 363. Printing the image of an object directly on the paper. - 

 With the apparatus set up exactly as for drawing or for printing 

 enlargements, one can expose the developing photographic paper to 

 the sharply focused image of the specimen. Of course this will give 

 a negative image, all the lights and shades being reversed, but the 

 outlines and proportions are perfect. Such pictures serve as useful 

 a purpose as shade-correct pictures for model making and for keeping 

 a record of one's specimens. 



PHOTOGRAPHIC REPRESENTATION OF VISUAL APPEARANCES 

 PANCHROMATIC PHOTOGRAPHY WITH "COLOR SCREENS 



364. Five methods of rendering objects visible. 



(1) The mounting medium and the object must have different 

 refractive indices, then the outline of the object or of its details are 

 margined by dark borders ( 137, refraction images). 



(2) The object or its details must have a different color from the 

 surrounding medium or neighboring objects (color images, 137). 



(3) The object or its details must appear self-luminous, the sur- 

 rounding field being dark (method of dark-ground illumination, 

 117). For large objects, an illuminated clock face on a dark night 

 is a good illustration. 



(4) If reflected light is used some parts of the object must absorb 

 the light and some parts reflect it; the different parts will then appear 

 as light and dark. 



