250 Proceedings of the Royal Irish Academy. 



the diffracted pencils admitted by the objective. If the admission of 

 these spectra is interfered with either by the limitation of the angular 

 aperture of the objective used from its original construction, or by 

 intentional screening off of any of the diffracted pencils which would 

 otherwise reach the final image of the object, the apparent detail will 

 be modified in a corresponding manner. These facts may be very 

 readily and simply demonstrated, when a microscope furnished with 

 an objective of one quarter of an inch equivalent focus is directed 

 npon a valve of PJeurosigma BaUicum, for example. If the objec- 

 tive has an aperture of 90° or thereabouts, there will be seen, after 

 focusing and removing the eye-piece in order to look down the tube of 

 the instrument, a brilliant image of the mirror or source of illumina- 

 tion, and symmetrically disposed round this direct image, if it occu- 

 pies the centre of the tube, four similar but much fainter images, 

 coloured red at their outer and fringed with blue at their inner edges, 

 if wliite light be used. 



These faint images are the diffraction images of the source of light, 

 each being composed of separate monochromatic, and individually 

 accurate, representations of the luminous origin, which are distributed 

 along a radius, commencing at the dii-ect image, and arranged in order 

 of increasing wave length. The composite image is therefore some- 

 what distorted, being elongated in a radial direction, and fringed 

 with colour as above described, from the overlapping of the extreme 

 images. 



If the luminous soui'ce be a narrow slit allowing a sufficiently 

 bright pencil to pass through a system of diffracting lines parallel to 

 itself, some of the Traunhofer lines may be seen, if solar light be 

 employed, especially if the eye be assisted by a magnifying glass 

 focused on the diffraction images. 



If now we conceal any two opposite spectra by appropriate screens, 

 and replace the eye-piece, we shall find that one of the systems of lines 

 with which the object is apparently marked has disappeared, namely, 

 that which is at right angles to the line joining the concealed spectra. 

 If while we still hide the same two spectra we also block out the 

 direct beam, the system of lines last mentioned will be replaced by 

 another, the components of which ai'e at half the distance of those 

 just described, and twice as numerous. 



This fact points the way to the explanation of the varying pheno- 

 mena, which attributes the visible systems of lines seen in the image 

 of such an object as we have chosen to the interference of the rays 

 from the images of the source of light formed in the upper focal plane 

 of the objective, where they meet within the eye-piece. 



The process of production of these interference strise is as follows ; 

 tracing the course of the light rays from the origin upwards, and 

 assuming them to be parallel. 



In the annexed diagram (1), let a, b be the incident beam falling 



1 Vide Xote added in Press. 



