ULTRAMICROSCOPY 59 



The value of the expression can also be diminished 



r 2/1 sin a 



by using light of short wave-length. Passing along the 

 spectrum from the red end towards the violet, the wave- 

 length diminishes, until at the extreme violet it is only about 

 half as long as in the red. Continuing beyond the range 

 of the visible spectrum in the same direction, the ultraviolet 

 is reached, which, though not affecting the optical nerves, acts 

 on a photographic plate and gives rise to fluorescence in 

 certain substances. In virtue of its shorter wave-length, it 

 can be seen that this will give a higher resolving power 

 than can be obtained with light of the visible spectrum. As 

 a source of light the electric arc between two metal electrodes 

 is commonly used ; electrodes of magnesium or cadmium are 

 very useful, since the light emitted is fairly homogeneous. 

 Since glass absorbs ultraviolet light, the lenses and other 

 portions of the apparatus must be made of quartz ; further, 

 the methods of observation must depend either on photography 

 or fluorescence ; the latter will diminish the accuracy and both 

 increase the experimental difficulties. 



The use of ultraviolet light allows us to obtain the form 

 of objects which are only about half as large as those which 

 can be examined with the light of the visible spectrum. 1 It 

 will therefore be possible to separate the images of two 

 objects separated by a distance of 1 x io~ 5 cm. or 0'i fi. 



The Ultra microscope. — By using the ultramicroscope, however, 

 much smaller objects can be seen, though as will appear later 

 their form cannot readily be obtained. It has been shown 

 that two point sources of light which are too close together 

 cannot be seen separately with the microscope ; and similarly, 

 details of structure cannot be recognised, if an object is too 

 small, when the microscope is used in the usual manner, and 

 transmitted light is used. Now there are some ultramicro- 

 scopic objects which can neither be studied nor even seen 

 under these conditions ; by using the ultra-microscope, we are 

 enabled to see them but not study them in an}' detail. We will 

 now pass on to consider how these ultramicroscopic particles 

 can be rendered visible. 



Consider an object, self-luminous and sufficiently far re- 



1 Wave-length of sodium light is approximately 6 x io -5 cm., of ultraviolet 

 light 2 - 3 x io -5 cm. 



