106 PROCEEDINGS OF THE SOCIETY. 



for specialisation, standardisation and repetition production, but design 

 came first. A thorough overhaul of design without reference to tradi- 

 tional designs must lead to startling results. 



Dr. Charles Singer, F.R.M.S., contributed a paper in which he 

 outlined " The Earliest Steps in the Invention of the Microscope." 



The author dwelt not on the well-known work of the classical 

 observers Leeuwenhoek and his successors, but he reverted to the 

 earliest stages in the discovery of the microscope, beginning with the 

 work of Euclid, and passing through that of Ptolemy and Alhazen to 

 Roger Bacon, who was truly the father of microscopy. Jansen of 

 Middle burg is usually regarded as the first to construct an actual micro- 

 scope, with Lippershey of Wesel as a rival, but Galileo was the first 

 effective discoverer of the microscope as of the telescope. 



Professor Alfred W. Porter, D.Sc, F.R.S., spoke on " The 

 Resolving Power of the Microscope." 



The paper consists in part of a brief historical summary. Emphasis 

 is laid on the entrance of the human element into the question of 

 resolving power. It is the " thing seen " with what we have to do ; 

 and no two people can see precisely alike. Nor can any unique limit be 

 fixed applying indifferently to various shapes of object and various 

 modes of illumination. Under best conditions two lines will not be 

 resolved by a dry objective unless they are more than half of a wave- 

 length apart. Even then they will not be seen as separate unless the 

 magnification reaches such a value that the ultimate image subtends at 

 the eye an angle greater than two minutes' of arc ; and if it is desired 

 to detect their separation with ease, an angle considerably greater than 

 this is desirable. 



More attention should be paid to the size of the Ramsden circle (the 

 bright spot), which is small in most microscopic cases. The image 

 probably begins to deteriorate in quality, owing to the reduction in the 

 beam entering the eye, when the total magnification with a one- twelfth 

 is about 300. The eye, however, is a fairly long-suffering organ. In 

 the paper it is throughout assumed that the optical system is perfect 

 from the point of view of geometrical optics. This is never the case ; 

 and improvements in this respect must be made pari passu with increase 

 in numerical aperture. 



Professor A. E. Conrady contributed some " Notes on Microscopical 

 Optics," which were communicated by Professor A. W. Porter. - 



The paper deals vj'idi the points in which the actual construction of 

 a lens may be improved. The-defects considered are spherical and 

 chromatic aberration, the secondary spectrum (which requires fluorite 

 glasses for its correction), and curvature of the field. It is indicated 

 how it may be possible to combine moderate curvature of field with 

 apochromatic perfection and thus remove the outstanding defect of the 

 best objectives. Defects in condensers are also discussed and remedies 

 suggested. 



