XI. WHEN TO USE SPECIAL MICROSCOPES 353 



length radiation the optics of the microscope and the illuminating 

 apparatus must be of quartz or other transparent material. Liquid 

 filters, Christiansen filters, or a monochromator are used to isolate 

 the desired wavelength. Much of the work has been done at 257 

 and 275 m/x with monochromatic objectives {20,21), although both 

 longer and shorter wavelengths have also been used. 



Since the eye is insensitive to these wavelengths, focusing of the 

 microscope must be done indirectly and the results observed from 

 photographs or from energy measurements on isolated parts of the 

 image with a photocell or other sensitive instrument. Originally a 

 fluorescent finder-ocular was used for focusing, or else a number of 

 pictures were taken by trial and error. The recent improvement of 

 fluorescent screens makes possible direct focusing in a darkened room 

 when the observer's eyes are dark-adapted, thus insuring good photo- 

 micrographs with a minimum of difficulty {22). The mounting 

 medium, shdes, and cover glasses must be transparent to ultraviolet. 

 Living tissues are damaged by short wavelength ultraviolet so that 

 exposures must be kept to the minimum. The eyes of the observer 

 must be protected from the radiation or a severe conjunctivitis will 

 result. 



Crystals just smaller than can be seen with light have been photo- 

 graphed for measurement and study of their form with the longer 

 wavelength radiation at 365 m^t. Since glass is transparent to this 

 radiation the ordinary microscope may be used {23). 



The catoptric microscope has also been used for ultraviolet micros- 

 copy (Sect. L2) and Brumberg {19) has described the use of a reflect- 

 ing objective, with revolving filter discs, fluorescent screen, and means 

 to convert the ultraviolet into visible light so that the object is seen 

 in color according to the position and nature of its ultraviolet-absorb- 

 ing bands. 



The ultraviolet microscope has been used to examine fine crystal- 

 loids, silver halide grains, nuclear detail in tissue cells with relation 

 to the question of genes and enzyme formation, location of proteins 

 and their concentration changes, regenerating nerve fibers, muscular 

 dystrophy, bacteria and virus particles, latex, rubber, and emulsions. 

 Some of the work is done with direct photographs, but the present 

 tendency is to use the instrument as a spectrophotometer to actually 

 measure the absorption bands of the specimen. Semiapochromatic 

 objectives are being designed to focus simultaneously several wave- 



