HOW TO USE THE MICROSCOPE 27 
sunlight is obtained, it will be found suitable for microscopic 
work. In no case should direct sunlight be used, because it 
will be found blinding in its effects upon the eyes. Natural 
illumination—diffused sunlight—varies so greatly during the 
different months of the year, and even during different periods 
of the day, that individual workers are resorting more and more 
to artificial illumination. The particular advantage of such 
illumination is due to the fact that its quality and intensity 
are uniform at all times. There are many ways of securing 
such artificial illumination, no one of which has any particular 
advantage over the other. Some workers use an ordinary gas 
or electric light with a color screen placed in the sub-stage 
below the iris diaphragm. In other cases a globe filled with a 
weak solution of copper sulphate is placed in such a way be- 
tween the source of light and the microscope that the light is 
Fic. 30.—Micro Lamp 
focused on the mirror. Modern mechanical ingenuity has de- 
vised, however, a number of more convenient micro lamps 
(Fig. 30). These lamps are a combination of light and screen. 
In some forms a number of different screens come with each 
lamp, so that it is possible to obtain white-, blue-, or dark-ground 
