466 THE ANATOMY OF WOODY PLANTS 



The photographic dark room should be of moderate size and 

 provided if possible with electric lights. In addition to a free 

 light there may be one shaded by a ground-glass cover for examin- 

 ing negatives and exposing lantern slides. Two safe lights for 

 developing are likewise necessary. One of these should be screened 

 with orange and deep-ruby glasses, one behind the other. This 

 light is available for the development of the less sensitive chro- 

 matic plates, known as iso-plates, and for all ordinary photo- 

 graphic plates. A second safe light is necessary for use with plates 

 sensitized by isocyanin. (so-called spectrum and panchromatic 

 plates), which must ordinarily be developed in total darkness. 

 The Wratten & Wainwright Company, of Croyden, England, has 

 patented a light which may be used for this purpose, and this is 

 on sale in the better photographic supply houses in the larger 

 American cities. It is indispensable for all highly color-sensitive 

 plates. A sink of good length and with two taps should be provided. 

 To the left there should be a gently sloping draining-board with 

 shelves beneath and above for developing- and fixing-trays of 

 hard rubber, as well as developers and glassware. Ventilation of 

 the dark room is desirable, and is absolutely essential in warm 

 weather. It is best effected by means of an ordinary electric fan 

 placed near the door of the room in such a position as to cause 

 strong currents into and out of the dark room. It may, of course, be 

 set in a special light-tight chamber communicating with the out- 

 side, but this fixed position considerably restricts its usefulness. 



The plates employed for making negatives depend on the par- 

 ticular needs in special cases. For very exacting work in which 

 the object is either extremely dark-colored or opaque the use of 

 panchromatic or spectrum plates is indicated. In case the con- 

 trasts are very slight, and particularly in weakly stained material 

 or objects which present only a slight natural coloration, the so- 

 called iso-process plate is valuable. For the mass of colored prepa- 

 rations the ordinary isochromatic plate answers every purpose. 

 A common need in connection with teaching is the copying of 

 illustrations. When these consist of line or half-tone engravings, 

 process plates giving a high degree of contrast are employed. It 

 was formerly necessary to import color-sensitive plates from 



