HI 



THE PRESIDENT'S ADDRESS. 



THE IMPROVEMENTS IN MODERN OBJECTIVES FOR 

 THE MICROSCOPE POPULARLY EXPLAINED. 



By E. J. Spitta, F.R.A.S., F.R.M.S., Etc. 



{Delivered February 11th, 1905.) 



Plates 9 and 10. 



Gentlemen, — 



The introduction of the Jena glass has been of so much 

 service to the computer of modern lenses, that the manufacture of 

 the old type of achromatic objective may be said to have ceased 

 to exist. At any rate, the improvement in definition and in colour 

 correction in the new so-called semi-apochromatic objective is of 

 so marked a character that I thought it might possibly interest 

 the members of this Club if for a few moments I attempted to point 

 out, in simple, untechnical language, in what way this new type of 

 lens construction differs from the old achromatic on the one hand 

 and the true apochromatic on the other. I have not the slightest 

 intention of bringing before you an exhaustive treatise on lens con- 

 struction, for that might not interest the majority of the members, 

 even if I felt myself competent so to do — which I most certainly do 

 not pretend for a moment. I wish merely, in a general way, to 

 point out the leading lines upon which the modern computer has 

 worked, and in what manner step by step he has so improved his 

 computations as to furnish us with a type of lens which is such 

 a decided advance upon anything hitherto constructed. I must 

 confess that to put this in simple language and yet to handle the 

 subject in a scientific manner has been a far more difficult task 

 than I anticipated, and one, I think, to which I should have 

 found myself unequal had it not been for the assistance of my 

 friend Mr. Conrady, whose knowledge of these matters is, both 

 practically and theoretically, so very extensive. 



Before actually commencing my subject, I may perhaps be 

 permitted to remind you of the interpretation and meaning of the 

 following terms, in case some of you may not have them quite 

 fresh in vour minds. 



