SCKEENS PLATES EXPOSUEE 215 



this has been suggested by Dr. E. J. Spitta, and is made by 

 Messrs. C. Baker of Holborn. It consists, firstly, of a Nernst 

 electric lamp, or some similar illuminant ; a very useful source 

 of light for this purpose is the 4-ampere arc lamp made by 

 Leitz, described on p. 106. The image of the source of light 

 is projected on to a slit by an achromatic lens ; the beam is 

 then collimated and passes through a prism with a grating 

 mounted on it, the resulting spectrum being projected on to 

 the sub-stage of the microscope by means of another achromatic 

 lens (Fig. 68). 



By moving the microscope from side to side, any portion of 



FIG. 68. Monochromatic Light Apparatus. (C. Baker.) 



the spectrum may be utilised as required. Provided that there 

 is sufficient dispersion to ensure that the whole of the field is 

 filled with light of one colour, this method is good enough. 

 But it must be admitted that the loss of light is very great 

 indeed, and in taking photographs the exposure is unduly 

 lengthened. Further, although the light may be visually 

 monochromatic, it is not even approximately of one wave-length, 

 as in the case of the mercury -vapour lamp. 



The use of deep-blue or violet filters is of importance not so 

 much for securing colour contrast, as for increasing the resolu- 

 tion of the microscope. Abbe's law for ascertaining resolution 



