PROCEEDINGS OF THE SOCIETY. 247 



the paper would be still better aware of what an elaborate work 

 Mr. Gordon had undertaken, and the great pains he had been at to 

 bring this subject completely before them. 



Prof. Wright, being called upon by the President to make some 

 remarks upon the subject before them, said he feared he had nothing 

 of any importance to add as he himself had sat at Mr. Gordon's feet 

 as a learner for a long time past ; but there were certain points alluded 

 to in this paper which struck him as being of direct practical interest 

 to all professional workers with the Microscope. The dimensions of the 

 beam which entered the eye appeared to be of great importance in con- 

 nection with ocular fatigue. Where the beam was large enough to 

 occupy to a large extent the opening of the iris — and this was the case 

 when a wide-angled objective and a low eye-piece were used — a person 

 could work all day long with the Microscope without fatigue. There 

 was no more ocular fatigue under these conditions than in looking at 

 objects in the ordinary way with the unaided vision. AVhere, on the 

 contrary, a narrow-angled objective and a high ocular were employed, 

 giving a very narrow beam, fatigue was very soon felt. Superadded 

 to ocular fatigue associated with the employment of a narrow beam, 

 there was of course the inconvenience resulting from obtruding spots 

 in the eye-piece and muscfe volitantes in the eye. The next point which 

 he felt it important for them to realise Avas the fact that the beam which 

 comes out of the eye-piece to enter the eye is inversely proportional to 

 the magnifying power employed. The initial size of the transmitted 

 beani depended of course on the aperture of the beam which was received 

 into the objective. The wide-angled objective derived much of its 

 importance from the fact that it furnishes the large initial beam which 

 was essential where high magnification was desired. The progressive 

 diminution of the beam as greater and greater magnification was achieved 

 had seemed to be of the nature of an insuperable difficulty. [This was 

 illustrated by a diagram on the board, showing that the opening out 

 and closing down of the terminal beam was by an action similar to that 

 of the lazy tongs, rigidly governed by the opening out and closing down 

 of the initial beam.] Mr. Gordon by his device of the interposed screen 

 had, so to speak, unhinged the lazy tongs, at the joint where the links 

 became unduly narrow, or if we choose to put it so, bent out the limbs 

 of the joint. He had in this way secured to us a wider emergent beam. 

 The last point to which he desired to refer related to the importance 

 of the step that was taken by Mr. Gordon when he cut himself loose 

 from the ordinary optical diagram representing only the axial beam, and 

 took into consideration the case of beams traversing the objective 

 obliquely, and Mr. Gordon had satisfactorily shown by the demonstra- 

 tions now on the table before them that the elimination of these oblique 

 rays in the case where they were cut down by the edge of the post- 

 objective diaphragm, was a matter of enormous importance to the 

 achievement of critical definition. In ordinary bacteriological work — 

 carried on as it usually was with a wide-angled oil-immersion objective, 

 and a condenser of a somewhat similar aperture, used without the 

 immersion fluid — the conditions were in point of fact conditions that 

 allowed the oblique beams from the periphery of the field to pass 



