The President's Address. By E. M. Nelson. 159 



About eighteen years ago your Council attempted to carry out this 

 same idea of standardisation, and proposed that of 1 • 5 in. for the sub- 

 stage, and ■ 92 and 1 ■ 35 in. for the tubes of eye-pieces. 



It was a foregone conclusion, however, that those gauges could 

 never have been adopted by the trade, because, with regard to the 

 substage, we have seen that it had already been standardised in this 

 country, and manufacturers did not see any sufficient reason why 

 they should incur the expense and inconvenience of changing the 

 gauge of their substage for one that not a single maker was using. 



With regard to the two gauges for eye-pieces, the smallest, viz. 

 0- 92 for the eye-piece tube, made it too large to enter any Continental 

 Microscope ; 1 '35 in. was too small to give a maximum field with a 

 Wenham binocular, and too large for a small binocular to be sold 

 at a popular price ; manufacturers therefore could not adopt either 

 of these gauges. The thanks of the Society are due to Mr. Conrad 

 Beck for the assistance he has rendered to the Council in this matter. 



With regard to our accounts, Mr. Vezey most kindly consented 

 to act as Treasurer, and by doing so has enabled the Council to lay 

 before you the year's accounts duly audited as is customary at our 

 Annual Meeting 



1 S- 



The Aplanatic Immersion Front. 



We will now pass on to the Address ; and I am going to ask you 

 to kindly bear with me for a short time while I endeavour to explain 

 a few points which will conclude the subject already dealt with in 

 my two former Addresses. 



You will no doubt remember that the subject was divided into 

 three parts, and that one which was called the middle portion came 

 first, and the first portion second ; so this, the last, will be the only 

 one in its proper place. Before beginning, permit me to point out 

 that, as in my previous Addresses so also in this one, nothing either 

 new or startling will be brought before you, and the subject will be 

 treated, as before, in a practical rather than in an academical style. 

 To exhaust any one of these divisions more space would be required 

 than could reasonably be allotted to all three Addresses together; 

 therefore each must be regarded only as a very fragmentary pre- 

 sentation of the subject. 



The simplest part of all Microscope lens construction is the 

 aplanatic oil-immersion front ; it is, I fear, nevertheless very imper- 

 fectly understood by microscopists as a whole, or even by many of 

 those forming the brass and glass contingent. Strange to say that, 

 although so important, it has not, so far as I am aware, been dealt 

 with in the whole range of microscopical literature except in a 

 single instance, on which occasion it was so ably handled by Sir 

 G. Stokes that it would not now have been taken up again had the 

 point of view been the same. Sir G. Stokes' paper was one of the 



