i -1 S THE PRESIDENT'S ADDRESS. 



farther on. A lens corrected for the blue ray — an ordinary 

 photographic objective — is drawn in Fig. 11. Blue is seen to 

 have the shortest focus, because it is the preferred colour; green 

 and violet are joined up next ; yellow and ultra-violet follow, whilst 

 red, outstanding, has the longest focus of all. 



We shall find that in the best work, by making the com- 

 ponents very thin, or else by very carefully proportioning their 

 relative thicknesses, the computer can also produce, at any rate 

 very approximately, equality of focal length for the different 

 colours, which, it should be borne in mind, also means that the 

 differently coloured images will be of the same size. Such com- 

 binations, when seen at their best, are said to have all the serious 

 aberrations corrected in their first approximations. Indeed, such 

 a combination, well effected, is near enough perfection if the 

 objective be of small size and of relatively long focus, say of not 

 less than two inches ; but directly the computer tries to make an 



_ G*V Y^tlVK" 

 achromatisedforBLUE 



ORDINARY PHOTOGRAPHIC OBJECTIVE 



objective of high power on these lines, where strongly curved 

 lenses become necessary, a number of other imperfections soon 

 become apparent, which rapidly place a limit on his powers. 

 These secondary aberrations are as follows. 



1. If we test an ordinary achromatic lens of considerable 

 angular aperture, as previously detailed, with monochromatic 

 light — the preferred colour selected being yellow-green for visual 

 instruments, greenish-blue for photo-visual purposes, and blue- 

 violet for purely photographic work — we find that, although we 

 have cured the spherical aberration of the central and marginal 

 rays, by making them come to the same focus, the rays from 

 the intermediate zones of the lens do not do so, being gener- 

 ally found to be under-corrected. This defect is often called 

 secondary spherical aberration, but, with the computer, spherical 

 zones. It is the most formidable of all troubles with which the 

 optician has to deal, even in the case of objectives of moderate 

 power ; and it has to be corrected by combinations of over- and 



