74 HAMILTON HARTRIDGE ON 



Ms own researches into the image formation in the microscope, ' 

 a small circular aperture and a collimator were used to provide 

 the necessary plane-wave illumination. In this case the source 

 of light was placed behind (and therefore at an appreciable 

 distance from) the small circular aperture, which therefore 

 itself became the source from the point of view of his theory. 

 Even if he had focused an image of the source into the plane of 

 the small aperture, as in modern spectroscopic practice, that 

 would not have made the aperture behave like a source of light. 

 It would seem clear, therefore, that " the source " in this case is 

 definitely not the source of light, but a virtual- point source placed 

 at infinity from which the plane waves produced by the aperture 

 and collimator may be considered to have started (" virtual " 

 being here used in precisely the sam.e sense as that used when 

 speaking of the virtual magnified image produced by an eye- 

 piece). 



(3) For low-power work with the microscope Abbe advised 

 the use of daylight and the plane mirror. This restriction to 

 low- power work is not due to any fundamental difference in the 

 method of image formation in objectives of low and high power 

 (Abbe was most emphatic on this point ; " even fence poles," 

 he said, " are seen by the formation of images according to my 

 theory"), but is due to the artificial restriction to the angle of 

 plane-wave illumination which the method provides. (The use 

 of the phrase " angle of plane-wave illumination " may perhaps 

 require explanation. Imagine plane-wave illumination to be 

 proceeding to a specimen lying on the microscope stage from a 

 number of sources placed at infinity. If the number of sources 

 is so immense that they join together to form a hollow sphere of 

 light, then it is clear that although the illumination is everywhere 

 due to plane waves, since these come from infinity, still it is also 

 correct to express the extent of the illumination as seen from the 

 .specimen in angular measure.) 



(4) For medium powers Abbe advised the use of the concave 

 mirror in conjunction with daylight, and for high- power work 

 the employment of the uncorrected condenser which bears his : 

 name ; the object in both cases being to increase the angle of 

 illumination. He therefore regarded daylight as the ideal 

 illuminant, not only without the condenser, but also with it. 

 J^ow in neither case is the sky the light source, for the small 



