438 



Note on a New Modification of Double Colour 

 Illumination. 



By J. Rheinberg. 



{Read October 15M, 1897.) 



I should like to bring before your notice, this evening, a new 

 modification of double colour illumination suitable for high 

 power. 



Similarly as with low-power colour illumination, on the dark- 

 ground principle, one of the ordinary double colour discs, having 

 a central spot of one colour, surrounded by a ring of a strongly 

 contrasting, and in this case preferably complementary colour 

 (e.g. red centre and green periphery) is placed in the substage 

 condenser, and by means of the iris diaphragm the relative 

 proportions of the two colours are so regulated that on looking 

 through the microscope the light appears to be neutral tinted. 



But although the background appears neutral tinted, a suitable 

 object will be seen coloured, in fact coloured differently in its 

 various parts, as, according to their form and position, they will 

 pick up a preponderance of one or other of the two colours by 

 which they are illuminated. To give a single concrete example : — 

 It is possible to light up a diatom so that the secondary structure 

 may appear in one colour and the primary structure in another, 

 both being very distinct at the same time. 



Differential colour illumination by methods hitherto described 

 has been confined to the use of cones of light either greatly 

 exceeding the aperture of the objective used (viz. on the dark- 

 ground principle), or very much smaller than the objective 

 aperture (viz. on the diffraction system), but it will be observed 

 that the particular modification described this evening permits 

 of the use of the illuminating cone ordinarily employed. Each 

 microscopist may use his own favourite cone. 



It will also be observed that diffraction plays but a quite 

 subsidiary part in this method as far as the colour effects are 

 concerned, so that no untoward results on this score need be 

 feared. 



