464 THE ANATOMY OF WOODY PLANTS 



to reds. The most valuable type of plate for general photomicro- 

 graphic purposes is that treated with a blue anilin dye, isocyanin. 

 This treatment makes the emulsion sensitive to the red end of the 

 spectrum to a very large degree. Such plates are called "pan- 

 chromatic" or "spectrum" plates and are of the greatest value 

 in the photomicrographic reproduction of extremely difficult micro- 

 scopic objects, such as coal, highly tanniferous stems, roots, etc. 



Further aids to the microscope in the objective reproduction of 

 microscopic images are the so-called color screens. These are of 

 various hues, but need not be considered in detail, since only a few 

 of them are practically useful for photomicrographic purposes. 

 When a yellow screen is employed, for example, it shuts off to a 

 large extent the violet and ultra-violet radiations and as a conse- 

 quence gives the photographic plate greater efficiency in the repro- 

 duction of colors other than light blue. A further extremely 

 valuable result following the use of a yellow screen is the elimina- 

 tion of the contrast between chemical and visual foci mentioned 

 above as a disadvantageous feature distinguishing microscopic 

 lenses from those employed in ordinary photographic cameras. 

 This is a property of the utmost practical importance and makes 

 available lenses which would otherwise be quite useless. The 

 general effect of a yellow screen, then, is to make a plate relatively 

 more sensitive to radiations other than the violet ones and at the 

 same time to favor a more brilliant microscopic image as a result 

 of the elimination of the ordinary chemical or actinic rays. A 

 green screen is in certain cases highly useful, but the numerous 

 other hues which distinguish screens made for the appropriate 

 rendering of color are in general of less value in the practice of 

 photomicrography. 



The photomicrographic apparatus may advantageously con- 

 sist of an ordinary camera with long bellows, supported in a ver- 

 tical position on a counterpoised sliding back. The back may be 

 fixed at any height by means of window fasteners. The camera 

 is connected with the microscope by a collar which engages with 

 a corresponding collar on the tube of the microscope in such a man- 

 ner as to break the path of the rays of light. Reflections are pre- 

 vented by the blackening of the collar rings with matt drop black. 



