Theories of Microscopical Vision. By A. E. Conrady. 613 



the entire aperture of any optical instrument directed towards it. 

 It is therefore illogical to adopt this theory when, by manipulation 

 or choice of the condenser, only part of the aperture of the objec- 

 tive is filled with direct light; yet this is nearly always the case, 

 as hardly any objectives of the present day will bear the absolute 

 filling with light which a self-luminous object would produce. 

 One of the most obvious demands of the spurious disc theory is 

 therefore almost invariably neglected. 



Some years ago Mr. J. W. Gordon read a long paper before 

 this Society in which several curious objections, each of which was 

 put forward as fatal to the Abbe theory — objections which I shall 

 dispose of further on, in so far as this has not been done already — 

 were followed by an attempt at an extension of the spurious disc 

 theory. Mr. Gordon speculated on the effect which diaphragms 

 introduced between the objective and the eye-piece would produce 

 in the image of a luminous point, and invented the name " anti- 

 point " for the image thus obtained. 



As no serious microscopist would ever think of interfering 

 with the regular clear and round aperture of an objective, or, at 

 any rate, would only reduce it by a " Davis diaphragm," without 

 destroying its simple circular form, it is difficult to see how cases 

 could arise where an " antipoint " would have to be considered 

 instead of the well-known spurious disc resulting with a single 

 round aperture. But to those familiar with optical experiments 

 it should at once be clear that the real and only significance of 

 Mr. Gordon's " antipoints " is to be found in their identification 

 as the complete diffraction spectrum produced by the diaphragm 

 introduced behind the objective. For, in these experiments of 

 Mr. Gordon's, the Microscope objective takes the place of the 

 collimator of a spectrometer, by roughly parallelising the rays 

 from the luminous point. These rays then suffer diffraction by 

 the diaphragm, and the resulting diffraction spectrum is observed 

 through the eye-piece ; the slight convergence of the pencils pro- 

 duced by the objective rendering a separate telescope objective 

 (such as is used in the usual laboratory instrument) unnecessary. 



Mr. Gordon's " antipoints " Tare therefore simply the familiar 

 diffraction spectra under a new name. 



I will now proceed to deal with the Abbe theory. 



It would seem that at the time when he enunciated his theory, 

 Prof. Abbe was quite unaware of the use which was, even at that 

 time, being made of condensers in England ; anyhow, he tacitly 

 assumed a mode of illumination by daylight or diffused lamplight, 

 directed by the mirror and limited by suitable diaphragms, and 

 showed that the light thus transmitted to the object was of such 

 a nature that it could not account for the formation of any image 

 in the manner usually assumed ; for the cone of rays having its 

 apex at any one point of the object would obviously consist of 



Dec. 21st, 1904. 2 Cr 



