THE PRESIDENTS ADDRESS. 149 



under-corrected elements acting in concert and in suitable 

 sequence. 



2. If we next substitute monochromatic light of wave-lengths 

 different from that of the preferred colour, we shall find that for 

 them spherical aberration is not corrected, even in the first approxi- 

 mation, the defect being generally that of under-correction for red 

 and over-correction for blue. This kind of secondary aberration 

 is, in microscopical objectives, next in importance to the last 

 mentioned, and has been called by the late Professor Abbe 

 " chromatic differences of spherical aberration," which he further 

 pointed out could not be cared without the use of glass of different 

 properties from that which was then in existence. The outcome 

 of this remark was the starting of the Jena glass factory. 



3. If we proceed to test for differences of focal adjustment 

 with the different colours, we find that whilst light of nearly the 

 same tint as the preferred colour shows no sensible difference of 

 focus, light of considerably different colour does, and in a some- 

 what curious fashion : — If the lens be corrected for visual work, 

 yellow-green, the preferred colour, will be found to have the 

 shortest focus, as we have before explained ; the other colours, 

 focussing at greater distances, blending, and in curious pairs, bright 

 red being united with blue, and, at a still greater distance, 

 deep red with indigo or violet, as we see in Fig. 9. This has been 

 called the secondary spectrum, and is the most serious defect in 

 telescopic objectives, but ranks only as an indifferent third in the 

 manufacture of the microscopical objective. The earlier types of 

 achromats made with the old kinds of glass will be found great 

 offenders with regard to the correction of these secondary 

 aberrations. In these the spherical zones are often found to 

 be reasonably well corrected for the preferred colour, but the 

 chromatic differences of spherical aberration are generally so 

 serious as to greatly reduce the usefulness of this type — at any rate 

 for accurate work, and particularly where a large cone of light is 

 employed or oblique pencils are used. It is needless to say that 

 for high-class photomicrography these lenses are simply useless. 



As we have already said, the introduction of the Jena glass 

 has changed the position of all things optical, for now we find the 

 computer can produce objectives which are spherically corrected 

 for two colours instead of only one, whilst the other wave-lengths 

 are almost perfectly corrected ; so we find ourselves face to face 



