176 METHODS IN PLANT HISTOLOGY 



the safranin-stained portion of the specimen, the contrast will be 

 sufficient and the result will be more satisfactory than if the safranin 

 stain had photographed as pure black, because all detail in the vessel 

 walls would then have been lost. 



A booklet, Photomicrography, published by the Eastman Kodak 

 Company, includes a table showing the spectral absorption bands of 

 some of the stains used in botanical microtechnique. It also gives data 

 on the transmission spectra of the Wratten filmers ordinarily used in 

 photomicrographic work. Three other booklets published by the East- 

 man Kodak Company will be helpful though they do not pertain di- 

 rectly to photomicrography. These are, Wratten Light Filters, The 

 Photographij of Colored Objects, and Color Films, Plates and Filters for 

 Commercial Photography. 



The final focusing of the microscope should be accomplished after 

 the Hght filter is in place in the path of the light and close to the sub- 

 stage condenser. The plate or film holder should be inserted, the light 

 turned off, the dark slide removed from the plate or film holder, and 

 the exposure made by turning on the light and turning it off. Making 

 the exposure in this way obviates the necessity for using a shutter in 

 the set-up of equipment. During the time of the exposure the table 

 on which the camera is resting should not be touched and no one should 

 be permitted to walk across the floor. These precautions are necessary 

 to prevent a blurring of the image. The time of the exposure will have 

 to be determined by trial. After a correct exposure has been obtained 

 it will be possible to compute further exposures with different filters 

 from the factors given in the booklet, Photomicrography. 



In the preceding discussion it was assumed that a photomicrograph 

 was to be made at fairly high magnification. For photographs at lower 

 magnifications the ocular is removed from the microscope, and it will 

 probably be best to remove also the draw tube. Care must be exer- 

 cised to avoid reflections within the microscope. A tube of dull, black 

 paper may have to be inserted in the microscope. A microscope with 

 a large barrel is useful for low-power work. When using 48 mm., 

 32 mm., and possibly 16 mm. objectives it will be necessary to remove 

 the substage condenser of the microscope in order to get the complete 

 field illuminated. It may also be necessary to insert a piece of ground 

 glass in the path of the light to give even illumination. Critical illu- 

 mination will be lost in this way, but for low-power work it will not 

 matter. 



