86 Modern Microscopy 



is practically available for monochromatic illumination is 

 restricted. 



Another advantage gained by the use of monochromatic 

 light is, that as there is but one colour of the spectrum 

 used, objectives of high-class make of the achromatic form 

 are rendered practically equal to apochromatics by the 

 removal of the secondary spectrum — that is, any chromatic 

 aberration that may be present in the objective is annulled 

 by the monochromatic light with which the illumination 

 is effected. The more nearly the monochromatic light 

 which is used approaches to that ray for which the spherical 

 aberration in the objective is best corrected, the better will 

 be the resulting definition. In some cases, so advantageous 

 has this means of illumination proved, that, using two 

 objectives of the same power alternately, one an expensive 

 apochromatic, and the other an achromatic, it has been 

 difficult to tell which was being employed. 



True monochromatic light can be at the present time 

 obtained by means of prisms only, and a very excellent 

 apparatus is that designed by Mr. E. M. Nelson, and 

 manufactured by Mr. C. Baker, of 244, High Holborn, 

 London. It consists of prisms, slits, and condensing-lenses 

 so arranged as to afford every facility for obtaining accurate 

 effects. 



Beautiful results with monochromatic light are to be 

 secured by the use of a heliostat to reflect sunlight upon a 

 slit and prisms. 



Dr. G. Johnstone Stoney has devised a most excellent 

 heliostat actuated by means of a lever clock movement, and 

 with fine adjustments by means of Hook's handles for 

 setting, and a plane-parallel worked mirror. The brilliance 

 of illumination secured by such means is very intense, and 

 a spectrum 2 feet in length can be easily secured if the 

 microscope be placed at some distance from the prisms. If 

 the spectrum be allowed to fall on a sheet of white paper or 

 cardboard at the position in which the microscope is placed, 

 the exact tint of illumination that is required can be selected, 



