PROCEEDINGS OF THE SOCIETY. 
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desirability of greater height of the base ; but then there was a veto 
against it being made otherwise. The German Microscopes were made 
of a certain height, and, of course, the English ones must be made the 
same ! 
Mr. T. Charters White read his paper “ On a new Method 
of demonstrating Cavities in Dental and Osseous Tissues,” which was 
illustrated by specimens exhibited under the Microscopes in the room. 
The President said the Society was much obliged to Mr. White for 
his paper ; certainly his specimens bore out his remarks, and they were 
most beautifully shown. 
Mr. E. M. Nelson exhibted an enlargement of a photomicrograph. 
He did not approve of that kind of thing ; but, as it was done on the 
Continent, perhaps, if nothing of the kind was produced in England, it 
might be said that they were unable to make enlargements. 
Mr. E. M. Nelson read his paper “ On the Optical Principles of 
Microscope Bull’s-eyes,” illustrating the subject by drawings on the 
blackboard. 
The President thanked Mr. Nelson for the practical way in which he 
had dealt with a subject of great importance to all who worked with the 
Microscope. 
Dr. Dallinger said that the remarks and details which had been laid 
before them by Mr. Nelson might have seemed to be dry and hard ; but 
in reality they were of the most practically useful kind which could be 
brought under the notice of such a society as theirs. He had not only 
pointed out defects in optical construction, but also the way in which 
those defects might be corrected. All who worked much with the 
Microscope were aware that it was a matter of the utmost importance to 
get a condenser as far as possible aplanatic, not merely upon the grounds 
mentioned by Mr. Nelson, but because a condenser so constructed was 
of the greatest importance in order to bring out the best results of an 
aplanatic objective. He was very glad, therefore, to find that Mr. Nelson 
had brought his practical mind to bear upon the subject, and that he 
had not only shown them the defects of existing forms, but had put into 
the hands of opticians the means by which those defects might be 
corrected. 
Mr. Mayall thought that, for the honour of their theoretical 
opticians, it should be mentioned that the theory as to the passage of 
the rays of light through lenses was dealt with by Herschel in his 
well-known treatise on light in the ‘ Encyclopaedia Metropolitan,’ and 
in Coddington’s ‘ Treatise on the Reflexion and Refraction of Light ’ 
(1829-30) it was gone into in a most complete manner, and the 
transmission of rays in the case of the meniscus, and every other form 
of non-achromatic lens, was exhaustively dealt with. This treatise of 
Coddington’s should not be confused with the two editions of his work 
on * Optics,’ published earlier. The later treatise embodied some of 
the then most recent investigations in optics by Airy, Herschel, and 
others, and was still regarded as one of the most important textbooks on 
the subject. The formula for aplanatic foci, to which Mr. Nelson had 
referred, was generally assigned to Lagrange. Gauss and Listing had 
