PROCEEDINGS OF THE SOCIETY. 475 



The Secretary said they had received a small quantity of Zircon 

 sand for distribution amongst those Fellows of the Society who wished 

 to have samples. Applications for this were to be made in writing to 

 the Assistant Secretary. 



Attention was called to a new ^\ { in. objective which was being ex- 

 hibited in the room by Mr. T. Powell. 



Mr. J. W. Gordon then read his paper entitled " A.n Examination of 

 the Abbe Diffraction Theory of the Microscope." The diagrams in 

 illustration of tbe subject were shown upon the screen, and the follow- 

 ing experiments were exhibited under a number of Microscopes on the 

 table : — 



Experiments Nos. 1, 2, 3, and 4 exhibited tbe phenomena described 

 by Prof. Abbe, as illustrated by the well-known apparatus supplied 

 by Carl Zeiss, of Jena, including tbe use of Abbe diaphragms with 

 single apertures of varying sizes and a triple aperture. 



Experiment No. 5. — In this experiment a diaphragm with two slots 

 was substituted for tbe Abbe diaphragm with three slots. Tbe spacing 

 of the slots was the same as in tbe Abbe triple-slotted diaphragm, and 

 the diffraction pattern produced by the diaphragm itself was therefore 

 similar, except as to brightness. Tbis diapbragm, which transmitted 

 those diffraction images tbat the triple Abbe diaphragm excluded, 

 yielded typically the same image, tbe appearances differing only in 

 brightness of illumination from those produced by the Abbe triple- 

 slotted diaphragm. 



Experiment No. 6. — The Abbe arrangement was modified by the 

 substitution of a broad uncovered flame as the source of light for the 

 Abbe substage-stop, in order to show that the diffraction images in the 

 principal focal plane take the shape of the source of light, but that 

 the details in tbe image are not sensibly modified, except in respect of 

 illumination. 



Experiment No. 7. — An Abbe Diffractions Platte and an aerial image 

 of a coarsely-ruled surface were shown in the same field ; the dimensions 

 of tbe image being adjusted so as to be visibly equal to those of the 

 Diffractions Platte, in order to show that, viewed through the Abbe 

 diaphragm, these two objects exhibit identical phenomena. 



Experiment No. 8. — A diaphragm with 1000 openings to the inch 

 was substituted for the Abbe diaphragm, and a ruled surface, having 

 lines varying in spacing from 0-0065 to 0*026 was substituted for 

 the Diffractions Platte, to show that, with this arrangement, the Abbe 

 phenomena can be observed upon a very coarse ruling. 



Experiment No. 9. — The Abbe substage-stop was mounted so far 

 behind the condenser that its focussed image formed the source of light. 

 By working the condenser up and down, the distance from the Diffrac- 

 tions Platte of this source of light was varied, and the curvature of the 

 wave-fronts passing the grating was correspondingly altered. The result 

 in altering the angles of divergence of the diffracted rays was observed 

 by focussing the instrument upon the source of light, and it was thus 

 shown that, whatever the combination of diffracted rays received by the 

 Microscope, the appearance of the image is always the same. 



Experiment No. 10. — The object on the stage was ruled with lines 



