CHAPTEE III 



ILLUMINATION AND ILLUMINATING 

 APPARATUS. 



Monochromatic Light and Light Filters. 



Absolutely monochromatic light is a light of one refrangi- 

 hility- — that is, a colour of one uniform wave-length. As 

 used in microscopy, monochromatic light means light with 

 a small range of refrangibility, and it is important that its 

 function should be clearly understood. 



If white light is divided into its component parts by 

 means of a prism or a spectroscope, a regular band of 

 colours is produced, termed the spectrum, commencing with 

 red at one end, followed by orange, yellow, green, blue, 

 indigo, and finishing with violet. In physical optics light 

 is regarded as travelling in waves, the amplitude of each 

 of which is very small, compared with the wave-length — not 

 more than about 1 : 10,000. Now, the length of a light 

 wave varies according to the portion of the spectrum that 

 is used. At the extreme red end of the spectrum it 

 measures "76 /j.,* and the wave-length decreases through 

 the range of colours until at the extreme violet end it 

 measures '39 jn. From this it will be seen that nearly 

 double the number of waves of light would be oscillating 

 per millimetre with a violet light than with a red. 



The numerical aperture of an objective is increased by 



* A* = TTsW of a millimetre, and is called a micron. There are 74 to 

 .8 microns in the diameter of a human blood-disc. 



