86 P. CRISP ON MICROSCOPIC VISION". 



Jena, and having an aperture exceeding that of a dry lens of 180° by- 

 no less than 25 per cent.), the object which excited much interest at 

 their last meeting, viz., P. angulation with the whole of the central 

 dioptric beam stopped out, and shown by the diffraction images 

 only. The hexagons have entirely disappeared, and are replaced by 

 what look like round holes punched out of the groundwork of the 

 valve — an appearance which, it is very interesting to observe, had, 

 unknown to Mr. Stephenson, been previously calculated by a pupil 

 of Professor Abbe, who had never seen P. angulatum, and to whom 

 the problem was set as a pure mathematical and physical one ; his 

 solution of it even takes into account a peculiar semi-transparency 

 which the groundwork shows. 



This experiment, I may mention, disposes of another fallacy that 

 has its advocates, viz., that the central part of the objective is 

 essential to fine definition. 



In leaving the subject here, I think it will be admitted that it is 

 a very important one, and it is not a little surprising that so few 

 microscopists have hitherto appreciated its importance or the revolu- 

 tion it makes in microscopy.* 



* A reference to this subject would be incomplete without a mention of 

 Dr. Fripp, of Bristol, to whom the thanks of English microscopists are due, 

 for having, at the cost of much labour, presented them with a translation of 

 Prof. Abbe's original communication in addition to numerous papers of his 

 own, written with the view of further elucidating the subject. 



