150 THE PRESIDENT'S ADDRESS. 



with an objective free from the chromatic differences of spherical 

 aberration of which I have just made mention. These new 

 objectives, if well made, should show no defect except that of 

 the pure secondary spectrum. They appear in the market 

 under many names — the achromatic of Zeiss, known as their 

 " Improved type," being the same as the semi-apochromatic& 

 of other makers. Sometimes we have entirely new names, such 

 as the " Holoscopic " of Messrs. Watson & Sons, or the " Modern 

 achromatic' 3 of Messrs. Powell & Lealand and others. This 

 new type is really a most excellent lens for all general purposes, 

 but, of course, for photomicrography it requires the use of a 

 suitable screen because of the secondary spectrum, and with 

 diatoms the image shows colour. This secondary spectrum is 

 entirely eliminated in the best type of apochromatic objective, 

 which makes it rank as the highest achievement of modern 



R-Y*B 



COLOR PARTcrAPOCHROMATIC SYSTEM 



r///rs£ colors united 



computation. I suppose this must be called the magnum opus 

 of Professor Abbe : but, really, he was such a genius in so 

 many different directions that I scarcely like to say that any 

 one result of his fertile brain was the best thing he ever 

 did. In the apochromatic, then — to repeat — besides the cure 

 of spherical zones and chromatic differences of spherical aberra- 

 tion, the secondary spectrum is entirely eliminated, leaving only 

 a very faint tertiary spectrum outstanding. The best types of 

 these remarkable lenses show hardly a trace of colour under 

 any conditions. I should further state that as three colours 

 are united, instead of two as in the semi-apochromatic objective, 

 photographs can be taken in light of any wave-length without 

 alteration of focus or adjustment. If we glance at Fig. 12 the 

 improvement is diagrammatically represented. I should like to 

 make one more remark before quitting the subject of apo- 

 chromatics— namely, that it should be borne in mind that, just 

 as the indiscriminate use of the Jena glass does not of necessity 

 render a lens worthy of the name of semi-apochromatic, so the 



