154 PHOTOMICROGRAPHY 



micrography, and it is essential that their action should 

 be clearly understood in order that the best selection 

 should be made for whatever object is under examination. 



Absorption Spectra Monochromatic Light. The spectra 

 of most radiants consist of a continuous band of 

 colours violet, blue, green, yellow, orange, and red 

 grading the one into the other, but if the light is passed 

 through a piece of coloured glass or through a col- 

 oured liquid, and then examined by means of a spectro- 

 scope, the spectra are found to have quite a different 

 appearance. They are no longer continuous, but broken 

 here and there by dark spaces or " Absorption bands." 

 The bands of colour still remaining depend on the nature 

 of the screen, and may be narrow and almost pure in 

 colour, or broader and show several colours ; they some- 

 times end quite sharply on one or both sides, but in other 

 cases gradually fade away. These bands of colour con- 

 stitute the " Transmission bands " of the filter in ques- 

 tion, and the spectrum made up of coloured bands and 

 dark spaces is called the "Absorption spectrum " of the. 

 dye or colouring matter ; if it consists of a single narrow 

 coloured band in any region, the light transmitted by 

 the screen is " Monochromatic." Strictly speaking, this 

 term should refer to light of one wave length, giving a 

 spectrum consisting of one line only, but this is not often 

 obtained in practice, except when the light from an 

 incandescent vapour is used, as in the Mercury Vapour 

 lamp. 



The Efficiency of Light Filters depends upon the par- 

 ticular rays of light that they respectively transmit or 

 absorb, upon the sharpness of the transmission band 

 or bands, and on the amount of light transmitted. In 

 regard to the last point a screen giving monochromatic 

 light, with very sharp demarcations between the trans- 

 mission band and the regions of absorption on either side, 

 would be of very little use if there was much loss of 





