G2t) 



NOTE. 



The Able Diffraction Theory. 



By J. W. Gordon, F.K.M.S. 



In Juno last 1 had the honour of submitting to the Royal Micro- 

 scopical Society a paper upon the Abbe theory of the Microscope. 

 The paper was devoted to an examination of that theory ; but at the 

 end of it I added a suggestion for the improvement of the imago by 

 the use, in a manner described, of a concave lens to spread the 

 transmitted wave-front. The suggestion was made to illustrate the 

 practical significance of the questions discussed, was confessedly im- 

 mature, and was submitted for further consideration and criticism. 



Since reading that paper I have myself seen cause to criticise that 

 suggestion adversely ; and in the hope of saving trouble to any 

 Fellows of the Society who may have thought the matter worth 

 following up, I take the liberty of communicating the criticism. 



It appears then, as the result of calculating out the effect of 

 introducing a concave lens into a Microscope in the manner suggested, 

 that improved definition will indeed result, but that incidentally the 

 scale of the image itself will be reduced, and that the diminution in 

 the diameter of the antipoint will be no more than proportionate to 

 this diminution in the scale of the image. This last result I did not 

 foresee : for, although I had made experiments before submitting 

 my paper, I had not made calculations to test the suggestion. The 

 experiments misled me, for I obtained better resolution with the 

 proposed adjunct than without it ; but the calculation leaves no room 

 for doubt that this was due only to facilitated manipulation, and that 

 with sufficient care I could have obtained the same resolution either 

 with or without the appliance. For, since the diameter of the anti- 

 point and the diameter of the image change in one and the same 

 proportion, the resolution of the image must remain unchanged. 





Dec. ISlli, 1901 2 u 



