XI. WHEN TO USE SPECIAL MICROSCOPES 351 



Other chemical pretreatment before staining. Most stains used are 

 relatively permanent, but even so, the preparations should be stored 

 away from light, especially sunlight. Some experien(;e and training 

 are necessary to interpi-et color contrasts and for the preparation and 

 examination of natural and artificially colored specimens. 



C. SPECIMENS THAT ABSORB NONVISIBLE RADIATION 



Longer and shorter wavelength radiation that is not visible is 

 useful for microscopy when absorbed by specimens. Special materi- 

 als are required for the construction of microscopes that are transpar- 

 ent to the invisible radiation used, likewise for the mounting of the 

 specimen. Since the image cannot be seen, it must be observed by 

 photography, photoelectric cells, or other indirect methods. This 

 involves understanding the use of special mechanisms for focusing 

 and recording the image. The microscopist must be familiar also 

 with the use and maintenance of the equipment required for produc- 

 ing the radiation used. For efficient use this may require considerable 

 training and skill. Even so, much useful information has been ob- 

 tained by the use of infrared and ultraviolet microscopy. 



1. Infrared Microscopy 



Ordinary microscopes transmit to a considerable extent in the 

 infrared region (X > 740 m^t). The resolving power of the micro- 

 scope decreases as the wavelength of radiation used increases. Most 

 infrared microscopy has been done with radiation of 800 to 1000 

 m/i. Tungsten lamps produce adequate amounts of this energy. 

 A filter is required to absorb the visible and shorter radiation and the 

 photographic film or plate must be sensitive to the infrared radiation 

 used. 



For the near infrared (800 m/x) the microscope may be focused 

 with a trichromatic red filter (Wratten A) and the Wratten 87 filter 

 substituted for it when exposing the picture. Other filters and 

 equivalent filters of other make are listed by Clark (18). Apochro- 

 matic objectives usually give better images in the infrared than 

 achromatic objectives. For focusing purposes in the longer wave- 

 length regions, a preliminary calibration curve may be made by ex- 

 posing a series of pictures at known positions of the fine adjustment. 

 Once the adjustment has been calibrated, the correct focus can be set 

 quite readily. Another method is to note the reading on the fine 



